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