Long-term solution for generalizing UI placement across frontends

[eritbh]

Related to #864, but even more general. There's a significant amount of stuff that we just never got working under jsAPI on new Reddit, but it seems like sh.reddit is probably going to have better adoption among mods than new.reddit did, so there will be more reason to get the rest of toolbox functionality working there. We're gonna need to build out a better system for generalizing the placement of UI elements across frontends, without the limitations that were inherent to jsAPI.