Blank root panel, asset form as a modal, and a dedicated Statuses page

- Main panel is now blank at the top level ("Select a folder or asset
  from the tree") instead of showing a dashboard - browsing happens in
  the tree, the panel only shows something once you click into it.
  Removed the now-dead DashboardStats/Stats() machinery it depended on.
- Add/Edit Asset no longer takes over the main panel; it opens in
  #assetModal instead; asset_form.html was trimmed down to just the form
  (no more breadcrumb/panel wrapper) since the modal chrome provides the
  title and footer buttons now. The dynamic item-type-fields and submit
  listeners moved from #main-panel's delegation to the modal element's.
- Statuses split out of the General settings page into its own
  /settings/statuses page and sub-nav entry, alongside General, Item
  Types, and Users.

web/templates/partials/modals.html

@@ -77,6 +77,23 @@
   </div>
 </div>
 
+<!-- Add / edit asset -->
+<div class="modal fade" id="assetModal" tabindex="-1">
+  <div class="modal-dialog modal-lg">
+    <div class="modal-content">
+      <div class="modal-header">
+        <h5 class="modal-title" id="assetModalTitle">New Asset</h5>
+        <button type="button" class="btn-close" data-bs-dismiss="modal"></button>
+      </div>
+      <div class="modal-body" id="assetModalBody"></div>
+      <div class="modal-footer">
+        <button type="button" class="btn btn-outline-secondary" data-bs-dismiss="modal">Cancel</button>
+        <button type="submit" form="assetForm" class="btn btn-primary" id="assetModalSubmit">Create Asset</button>
+      </div>
+    </div>
+  </div>
+</div>
+
 <!-- Move asset -->
 <div class="modal fade" id="moveAssetModal" tabindex="-1">
   <div class="modal-dialog">