Replace sidebar with a tab bar

This is phase 1 of a larger change to restructure the overall user experience. It replaces the "basement" sidebar with tabs (#333). A tab bar will allow us to grow the app in new directions more easily.

A very large amount of code has been changed and removed. This is considered a risky pull, and there'll definitely be some issues. I advise we merge it nonetheless to minimize conflicts with future merges.

The app still doesn't look great on iPad. A separate pull will be forthcoming to address that. This pull merges a couple other pulls as well that had overlapping changes. Hopefully this makes the merge easier, not trickier.

Video:
https://cloudup.com/cZ1Mecfvo4y