build: the themes ship compiled, libsass is no longer installed

A theme change compiled Bootswatch with libsass (34 MB, upstream
deprecated) into two tracked files, which updates, rollbacks and new
Docker containers then reset. The 27 themes now ship as
static/css/themes/<theme>.min.css, built by "manage.py build_themes";
the Settings page only saves the theme name and every page links that
file (zephyr when no settings are in context, as on the 500 page).

libsass moves to dev/requirements.txt; a test checks that each shipped
file is its theme compiled from dev/scss. The "Theme SASS Path" setting
is removed, and the theme default "flaty", a typo that never applied,
becomes zephyr, the theme those installs showed. The update and
rollback steps restore an older version's rewritten theme files only
where they are modified.

README.md

@@ -457,7 +457,7 @@ Move the `environment=` values of that config into `/etc/default/webvirtcloud` (
    { [ ! -f /etc/default/webvirtcloud ] || cp -p /etc/default/webvirtcloud "$b"/; } &&
    git rev-parse HEAD > "$b/commit" &&
    git ls-files -z --others --exclude-standard -- '*/migrations/*.py' | tar --null -T - --remove-files -cf "$b/local-migrations.tar" &&
-   git checkout -- static/css/wvc-main.min.css dev/scss/wvc-main.scss &&
+   git checkout -- $(git ls-files -m -- static/css dev/scss) &&
    git checkout master && git pull
    ```
 
@@ -537,7 +537,7 @@ b=/root/webvirtcloud-backup-YYYYMMDD-HHMMSS
 systemctl stop webvirt-cloud webvirt-novncd webvirt-socketiod webvirt-worker
 cd /srv/webvirtcloud &&
 ! systemctl is-active --quiet webvirt-cloud webvirt-novncd webvirt-socketiod webvirt-worker &&
-git checkout -- static/css/wvc-main.min.css dev/scss/wvc-main.scss &&
+git checkout -- $(git ls-files -m -- static/css dev/scss) &&
 git checkout "$(cat "$b/commit")" &&
 tar -xf "$b/local-migrations.tar" &&
 venv/bin/python3 -c "import os,sqlite3,sys; [os.remove(p) for p in ('db.sqlite3-journal','db.sqlite3-wal','db.sqlite3-shm') if os.path.exists(p)]; sqlite3.connect(sys.argv[1]).backup(sqlite3.connect('db.sqlite3'))" "$b/db.sqlite3" &&
@@ -589,7 +589,7 @@ A version from before v3.0.0 has no worker: `systemctl disable --now webvirt-wor
 - A running VM without a guest agent shows the IPv4 address the host learned by ARP (the lookup always failed before).
 - After a migration the VM's NVRAM file is deleted on the source, when it is in libvirt's `/var/lib/libvirt/qemu/nvram` and the destination does not share that directory (a VM created there later with the same name would start with those UEFI variables).
 - gstfsd finds the guest's operating system the way libguestfs inspects it, so a root on LVM works, and it always answers (an error when it finds no or several operating systems). Update `/usr/local/bin/gstfsd` on every compute (see [Adding a Compute Node](#adding-a-compute-node), step 3).
-- A theme picked on the Settings page rewrites two tracked files; the update and rollback steps put back the shipped theme. Pick yours again afterwards, also after Docker recreates the container.
+- The themes ship compiled in `static/css/themes`; picking one on the Settings page only saves its name, so updates, rollbacks and recreated Docker containers keep it, and the "Theme SASS Path" setting is gone. libsass is no longer installed; `manage.py build_themes` recompiles the themes in a development setup.
 - With the bundled nginx config, browsers revalidate static files on each load (304 when unchanged), so an update or a theme change shows without a hard reload. Files cached before this update can stay until a hard reload (Ctrl+F5) of each affected page, or until the site's browser cache is cleared once; an external caching proxy may need a purge.
 - External snapshots use libvirt's snapshot API and need libvirt 9.9 or newer on the compute hosts. They work on UEFI VMs and running, paused or shut-off VMs, can save memory too or, on a running VM with a connected QEMU guest agent, freeze the guest's file systems (the default there), and a revert keeps disks added later (it detaches them). A snapshot is refused when a storage pool with the VM's disks has less free space than a share of those disks (10%, at least 1 GiB; both on the Settings page) plus, for the pool of the memory file, the VM's memory; this is checked before the snapshot only, nothing is reserved. A VM's snapshots form one chain: after a revert, delete the newer snapshots before taking another. Snapshot names no longer get an `s1.` prefix; existing ones keep theirs and can still be reverted and deleted, except that a snapshot an older version took of a UEFI VM can only be deleted. On NFS, libvirt removes merged snapshot files as the qemu user, who needs write access to the image directory; files it could not remove are named in a warning after the delete or revert. Deleting a VM with its data removes its snapshot files too, except a template it grew from and files another VM uses.
 - A storage volume a VM still needs (a disk, a backing file or a snapshot's file) is no longer deleted from the storage page or the API; the delete is refused with the reason. Only VMs on the same compute are checked.