refactor(templates): Bootstrap utilities instead of the upstream rules that redid them

webvirtcloud.css still carried the upstream panel's rules for what
Bootstrap 5.3 has as utilities and components:
- the page's bottom space (body padding) is pb-5 on base.html's body,
  and the console page no longer has to undo it;
- the VM page's action tabs (.action-button, .action-block with a
  fixed 65px width) are text-center with a d-block icon; its tab panes
  (#power ... #undefine, one rule each) mt-3;
- the bordered sub-tab panes (.tab-pane-bordered, a fixed #eee border
  that dark themes could not change) border border-top-0 p-3;
- the search box and header buttons (.search, .btn-header) m-1 and
  mt-1; the SSH key select (.keyselect) a form-select;
- the boot order list (.multipleselect, fixed size, #c0c0c0 border) a
  list-group of form-check-input items; its unused .multipleselect-on
  goes;
- table cells centred by a global rule are align-middle on each table.
Two rules against Bootstrap's behaviour go: menus opened on hover on
wide screens (Bootstrap opens them on a click, and keyboard and touch
users had no hover), and "a { outline: 0 }", which hid the keyboard
focus ring.

The paragraph and page-header margins and the 900px container stay
for a separate change: they set every page's layout.

computes/templates/computes/instances.html

@@ -8,7 +8,7 @@
 
 {% block page_heading_extra %}
 <a href="{% url 'instances:create_instance_select_type' compute.id %}"
-    class="btn btn-success btn-header float-end" title="{% trans "New Instance" %}">
+    class="btn btn-success mt-1 float-end" title="{% trans "New Instance" %}">
     {% bs_icon 'plus-circle-fill' %}
 </a>
 {% if instances %}
@@ -39,7 +39,7 @@
             {% endif %}
         {% else %}
             <div class="table-responsive">
-                <table class="table table-hover" data-sortable>
+                <table class="table table-hover align-middle" data-sortable>
                     <thead>
                         <tr>
                             <th scope="col">{% trans 'Name' %}<br>{% trans 'Description' %}</th>