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.

interfaces/templates/create_iface_block.html

@@ -13,8 +13,8 @@
                     <h5 class="modal-title">{% trans "Create New Interface" %}</h5>
                     <button type="button" class="btn-close" data-bs-dismiss="modal" aria-label="Close"></button>
                 </div>
-                <div class="modal-body">
-                    <form method="post" action="" role="form" aria-label="Create New Interface form">{% csrf_token %}
+                <form method="post" action="" role="form" aria-label="Create New Interface form">{% csrf_token %}
+                    <div class="modal-body">
                         <div class="row">
                             <label class="col-sm-3 col-form-label">{% trans "Name" %}</label>
                             <div class="col-sm-6">