Fix new logged-out "Login" button URI and workflowiness

Summary: Whelp apparently I never actually clicked this.

Auditors: btrahan
Cache the computation of the custom block component of the markup key

Summary:
Caught this taking way too long on a production profile:

https://secure.phabricator.com/xhprof/profile/PHID-FILE-vfzq3sregh5xvpf5nc2t/?symbol=PhabricatorMarkupEngine::getMarkupFieldKey

Cache it; it's always identical.

Test Plan: Loaded Conpherence locally.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7583
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