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 controller/strategy/ies anyway.

Also made PhabricatorRepository::getRemoteURIObject() public, because it was very useful in getting
the domain and path for the repo.

Test Plan:
Went through these flows:

• load revision in hosted, github-backed, non-github backed repos to see button as needed.
• hit land with weak token - sent to refresh it with the extra scope.
• Land to repo I'm not allowed - got proper error message.
• Successfully landed; Failed to apply patch.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T182

Differential Revision: https://secure.phabricator.com/D7555