Send Differential e-mails in user's language

Summary:
Works this way:

• Select users' language with multiplexing.
• Select default language otherwise (it can be different from current user's language).
• Build body and subject for each user individually.
• Set the original language after sending the mails.

Test Plan:

• Comment on a diff of user with custom translation.
• Set default to a custom translation. Comment on a diff of user with default translation.
• Set default to a default translation. Comment on a diff of user with default translation.

Repeat with/without multiplexing.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1139

Differential Revision: https://secure.phabricator.com/D2774