fix(templates): pages parse without markup errors, element IDs are unique

Rendered through every URL on libvirt's test driver, the pages had
duplicate IDs and markup browsers repaired silently:

- Each flavor modal's "Advanced" button toggled #flav-add-collapse, the
  first modal's section; every modal now has its own. The flavor and
  template copies of the custom tab's control IDs, and the template
  tab's unused images input, are gone.
- The VM page's action icons and the XML editors were styled by
  repeated IDs; they use classes.
- Modal forms opened inside modal-body and closed after modal-footer;
  they wrap both. Stray end tags and duplicate attributes are removed,
  paragraphs no longer hold modals or rows, table heads have a row, and
  the create page closes its row and column.
- html lang is the active language instead of "en".

A test walks every URL as a superuser and fails on a repeated ID. The
console page still has no doctype: its full-height layout relies on
quirks mode and needs its own check.

admin/templates/admin/user_list.html

@@ -53,7 +53,7 @@
                         </td>
                         <td>{{ user.last_login }}</td>
                         <td>{% if user.is_staff %}{% bs_icon 'check' %}{% endif %}</td>
-                        <td>{% if user.is_superuser %}{% bs_icon 'check' %}</span>{% endif %}</td>
+                        <td>{% if user.is_superuser %}{% bs_icon 'check' %}{% endif %}</td>
                         <td>{% if can_clone %}{% bs_icon 'check' %}{% endif %}</td>
                         <td>
                             <div class="float-end btn-group">