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/item_table.html

@@ -11,10 +11,9 @@
 				<th scope="col">Sub-total</th>
                 {% endif %}
 				{% if edit %}
-				<th scope="col" class="text-right align-self-start">
+				<th scope="col" class="text-end align-self-start">
 					<button type="button" class="btn btn-success btn-sm item-add"
-						data-toggle="modal"
-						data-target="#itemModal">
+						data-modal-target="#itemModal">
 						<i class="fas fa-plus"></i> Add Item
 					</button>
 				</th>
@@ -65,11 +64,11 @@
         <td class="quantity"></td>
         <td>£<span class="sub-total"></span></td>
         {% if edit %}
-        <td class="vert-align text-right">
+        <td class="vert-align text-end">
             <div class="btn-group" role="group" aria-label="Action buttons">
 	            <button type="button" class="item-edit btn btn-sm btn-warning"
 			            data-pk="{{item.pk}}"
-			            data-toggle="modal" data-target="#itemModal">
+			            data-modal-target="#itemModal">
 		            <span class="fas fa-edit"></span>
 	            </button>
 	            <button type="button" class="item-delete btn btn-sm btn-danger"