When we introduced sidebars initially, we had an area where a sidebar

could go on the page for desktop mode, and a separate area for mobile

mode. The mobile sidebar was a bit different, in that it had links for

logging in/out, changing settings, and going to Docs, Support, and

Admin.
It was up to each page to populate both sidebars, and styling wasn't

consistent across both. This made it a bit of a pain to manage both

types of sidebars, making it all more error-prone. The aesthetic of the

sidebar was also a bit old-looking, resembling the Review Board 2.0 UI a

bit too much. On top of that, our CSS transitions often broke when

opening the sidebar, making things look really sloppy.
This change fully redesigns the sidebar. There is now a single, standard

template block where sidebar content can go, housing either the

new-style or old-style CSS components for sidebar content. This defaults

to containing the Review Board navigation bar (New Review Request, My

Dashboard, etc.), followed by the page's sidebar content. Styling is

consistent (and improved if using the new CSS components), transitions

work correctly, and on top of it, the sidebar's look matches that of the

page theme (blue for Dashboard, brown for review requests, etc.).
This should mostly Just Work (TM) for any pages already making use of

sidebars, but those pages will definitely be improved, aesthetically, if

they try to use the new components. Going forward, we'll want to

eventually remove support for the old sidebar styling.
There are some areas in which things will not work, however. For

instance, the New Review Request page tries to move the desktop version

of the sidebar back into view for navigation purposes, which will not

work with the new design. As it was a bit of a hack on top of the

sidebar code, a more formal, future-proof solution will be needed, and

that will be tackled separately.
The elements are defined as a new .rb-c-page-sidebar component, which

provides standard styling and room for future expansion. This groups the

sidebar into multiple layers, one of which can be shown at a time.

Layers can contain a .rb-c-sidebar, or any other content as needed by

the page.
Some of the styling and the mixin functions for the new rb-c-sidebar

component have also been moved for this, allowing sidebars to be styled

independent of their place on the page. This will further evolve in

upcoming changes.

Tested all the current sidebars in mobile and desktop mode.
Tested this along with the upcoming sidebar work in the new Admin UI.