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

RIGS/templates/partials/event_details.html

@@ -11,7 +11,7 @@
                         {{ object.venue|namewithnotes:'venue_detail' }}
                     </a>
                 {% endif %}
-                {% if object.parking_and_access or object.riskassessment.parking_and_access %}<span class="badge badge-warning">Additional Access Requirements</span>{% endif %}
+                {% if object.parking_and_access or object.riskassessment.parking_and_access %}<span class="badge text-bg-warning">Additional Access Requirements</span>{% endif %}
             </dd>
             {% if object.venue %}
             <dt class="col-sm-6">Venue Notes</dt>
@@ -85,7 +85,7 @@
             {% else %}
             <a href="{% url 'event_thread' object.pk %}" class="btn btn-primary" title="Create Forum Thread" target="_blank"><span
             class="fas fa-plus"></span> <span
-            class="hidden-xs">Create Forum Thread</span></a>
+            class="d-none d-sm-inline">Create Forum Thread</span></a>
             {% endif %}
         </dl>
     </div>