Modularize Herald "flag" action, plus update transcripts

Summary:
Ref T8726. This modularizes "Mark with flag", plus rebuilds transcripts in a more modern/flexible way. The big transcript stuff is:

• Transcripts are now translatable.
• Transcripts can now show multiple outputs from a single action. For example, an action like "add A, B, C to subscribers" can now say "added A; B is invalid; C was already subscribed".

Test Plan: {F637784}

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: joshuaspence, eadler, epriestley

Maniphest Tasks: T8726

Differential Revision: https://secure.phabricator.com/D13649