Bootstrap 5 & remove jQuery (#661)

* build: move frontend assets to Bootstrap 5.3 and Tom Select

* feat: migrate templates and scripts to Bootstrap 5, dropping jQuery

Templates
- Rename Bootstrap 4 classes and data attributes (data-bs-*, ms-/me-/ps-/pe-,
  text-start/end, form-group -> mb-3, input-group wrappers removed, badges,
  custom controls -> form-check/form-select, media -> d-flex, btn-close, ...)
- Dark mode now uses Bootstrap 5.3 colour modes (data-bs-theme on <html>),
  set from <head> to avoid a flash of the light theme
- Remove the Bootstrap 4 /bootstrap/ demo page
- Contextual table classes (table-success etc.) still work as plain
  backgrounds for the rigboard key

JavaScript
- Remove jQuery: src.js/interaction.js and every inline template script are
  rewritten in plain JS on top of small helpers (helpers.js, bundled with
  Bootstrap into jpop.js so inline scripts can use them)
- Replace bootstrap-select/ajax-bootstrap-select with Tom Select. The remote
  search now sends the `q` parameter and reads `text`, which is what the
  API actually uses (the old picker sent `term` and read `label`)
- HTML fragments are fetched with an X-Requested-With header so views keep
  rendering their modal variants, and fetched scripts are re-executed
- Remove the Bootstrap 4 plugin script includes

* fix: stop rigboard header buttons stretching to full width under Bootstrap 5

* fix: restore dark-mode brand colours and contextual table colours under Bootstrap 5

* fix: brighten dark-mode links and make coloured card outlines visible

* fix: calendar layout and grid borders, navbar search button width, guard missing modal links

* fix: readable text in dark-mode Tom Select inputs and placeholders, restore form-control column widths

* build: split the Dockerfile into cached stages with prod and dev targets

* deploy: make compose.yml development-only with live reload, and document it

* fix: keep badge text colour for links inside badges

* fix: restore table-bordered grid lines in dark mode

* fix: include Bootstrap's progress styles and keep progress labels readable

templates/base_ajax.html

@@ -2,9 +2,7 @@
     <div class="modal-content">
         <div class="modal-header">
             <h4 class="modal-title">{{page_title|safe}}{% block title %}{% endblock %}</h4>
-            <button type="button" class="close" data-dismiss="modal" aria-label="Close">
-              <span aria-hidden="true">&times;</span>
-            </button>
+            <button type="button" class="btn-close" data-bs-dismiss="modal" aria-label="Close"></button>
         </div>
         <div class="modal-body">
             {% block content %}{% endblock %}
@@ -12,15 +10,5 @@
         <div class="modal-footer">
             {% block footer %}{% endblock %}
         </div>
-        <script>
-            $('.modal-content form').on('submit', function(e) {
-                e.preventDefault();
-                data = $(this).serialize();
-                action = $(this).attr('action');
-                $.post(action, data, function(resp) {
-                    $('#modal').html(resp);
-                });
-            });
-        </script>
     </div>
 </div>