FEATURE: Initial admin sidebar navigation (#24789)

This is v0 of admin sidebar navigation, which moves
all of the top-level admin nav from the top of the page
into a sidebar. This is hidden behind a enable_admin_sidebar_navigation
site setting, and is opt-in for now.

This sidebar is dynamically shown whenever the user enters an
admin route in the UI, and is hidden and replaced with either
the:

* Main forum sidebar
* Chat sidebar

Depending on where they navigate to. For now, custom sections
are not supported in the admin sidebar.

This commit removes the experimental admin sidebar generation rake
task but keeps the experimental sidebar UI for now for further
testing; it just uses the real nav as the default now.

lib/svg_sprite.rb

@@ -19,6 +19,7 @@ module SvgSprite
         archive
         arrow-down
         arrow-left
+        arrow-right
         arrow-up
         arrows-alt-h
         arrows-alt-v
@@ -171,6 +172,7 @@ module SvgSprite
         mobile-alt
         moon
         paint-brush
+        palette
         paper-plane
         pause
         pencil-alt