Use Log In vs. Login when it's a verb

Summary: Cursory research indicates that "login" is a noun, referring to a form, and "log in" is a verb, referring to the action of logging in. I went though every instances of 'login' I could find and tried to clarify all this language. Also, we have "Phabricator" on the registration for like 4-5 times, which is a bit verbose, so I tried to simplify that language as well.

Test Plan: Tested logging in and logging out. Pages feel simpler.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D18322