Centralize "this is the current user for the request" code

Summary:
Ref T11098. This primarily fixes Conduit calls to *.edit methods failing when trying to access user preferences.

(The actual access is a little weird, since it seems like we're building some UI stuff inside a policy query, but that's an issue for another time.)

To fix this, consolidate the "we're about to run some kind of request with this user" code and run it consistently for web, conduit, and SSH sessions.

Additionally, make sure we swap things to the user's translation.

Test Plan:

• Ran maniphest.edit via arc call-conduit, no more settings exception.
• Set translation to ALL CAPS, got all caps output from ssh and Conduit.

Reviewers: avivey, chad

Reviewed By: chad

Maniphest Tasks: T11098

Differential Revision: https://secure.phabricator.com/D16066