R1:dcf909ba56bf

Land to GitHub + support stuff

Summary:
A usable, Land to GitHub flow.

Still to do:
- Refactor all git/hg stratagies to a sane structure.
- Make the dialogs Workflow + explain why it's disabled.
- Show button and request Link Account if GH is enabled, but user is not linked.
- After refreshing token, user ends up in the settings stage.

Hacked something in LandController to be able to show an arbitrary dialog from a strategy.
It's not very nice, but I want to make some more refactoring to the…