FEATURE: Initial version of experimental admin search (#31299)

This feature allows admins to find what they are
looking for in the admin interface via a search modal.
This replaces the admin sidebar filter
as the focus of the Ctrl+/ command, but the sidebar
filter can also still be used. Perhaps at some point
we may remove it or change the shortcut.

The search modal presents the following data for filtering:

* A list of all admin pages, the same as the sidebar,
   except also showing "third level" pages like
   "Email > Skipped"
* All site settings
* Themes
* Components
* Reports

Admins can also filter which types of items are shown in the modal,
for example hiding Settings if they know they are looking for a Page.

In this PR, I also have the following fixes:

* Site setting filters now clear when moving between
   filtered site setting pages, previously it was super
   sticky from Ember
* Many translations were moved around, instead of being
   in various namespaces for the sidebar links and the admin
   page titles and descriptions, now everything is under
   `admin.config` namespace, this makes it way easier to reuse
   this text for pages, search, and sidebar, and if you change it
   in one place then it is changed everywhere.

---------

Co-authored-by: Ella <ella.estigoy@gmail.com>

plugins/discourse-local-dates/config/locales/client.en.yml

@@ -1,4 +1,9 @@
 en:
+  admin_js:
+    admin:
+      site_settings:
+        categories:
+          discourse_local_dates: "Discourse Local Dates"
   js:
     discourse_local_dates:
       relative_dates: