Get Toolbox working in sh.reddit

[eritbh]

The end goal here would be to get feature parity between "new new Reddit" (sh.reddit.com) and "new Reddit" (new.reddit.com, except as of recently that apparently also shows the sh.reddit design now). This would not implement features that are currently exclusive to old Reddit, only the ones that are currently available on new Reddit.

Implementation-wise, this will be like the oldreddit module, emulating jsAPI events and creating slot containers for buttons and stuff to go. This is gonna suck a lot more than writing the oldreddit module though given the architecture of sh.reddit.

[eritbh]

Scope here has changed significantly - this mostly just relies on #865 now. That issue covers the new system for frontend slots and frontend observers, and their parity with the old implementation on existing frontends; this issue covers the subsequent creation of the shreddit frontend observer to get things working on shreddit.