We have separated and combined modes for sidebar panels.
Separated means the panels show only their own sections,
combined means sections from all panels are shown.
The admin sidebar only shows its own panels, so it must set
the mode to separated; however when we navigate to chat or
home we must revert to the initial mode setttings.
When hiding/showing the sidebar, as is the case on mobile
and using the toggle in the top left on desktop, we delete
and recreate the ember component on the page. This causes
the `sections` for each sidebar panel to get re-evaluated
every time.
For the admin sidebar, this means that we were constantly
re-adding the plugin links to the sidebar, causing duplication.
This can be fixed by just adding @cached to the getter for
sections.
This changes the Plugins link in the admin sidebar to
be a section instead, which then shows all enabled plugin
admin routes (which are custom routes some plugins e.g.
chat define).
This is done via adding some special preloaded data for
all controllers based on AdminController, and also specifically
on Admin::PluginsController, to have the routes loaded without
additional requests on page load.
We just use a cog for all the route icons for now...we don't
have anything better.
