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.

templates/base.html

@@ -1,7 +1,8 @@
 {% load static %}
 {% load django_bootstrap5 %}
+{% load i18n %}
 <!DOCTYPE html>
-<html lang="en">
+{% get_current_language as LANGUAGE_CODE %}<html lang="{{ LANGUAGE_CODE }}">
   <head>
     <meta charset="utf-8">
     <meta http-equiv="X-UA-Compatible" content="IE=edge">
@@ -40,7 +41,7 @@
     {% endif %}
 
     <div role="main" class="container">
-      <div class"row">
+      <div>
         <div>
           <div class="float-end">
             {% block page_heading_extra %}