Consolidate User Profile loading

Summary:
Introduce `PhabricatorUserQuery::attachProfilesForUsers()` for batch attachment of user profiles (for a future diff)
Introduce `PhabricatorUser::loadUserProfile()` to load attached user profiles (or load them on their own, if no one is attached).

USed them in code

Test Plan:
verified that use sites did not break

- Uploaded user profile image for bots
- changed my own profile image and blurb
- looked a lot at my own profile

Reviewers: epriestley, btrahan, chad

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5395