R1:8b12fa6d6ed0

Prepare TransactionEditor for silent transactions via bulk edit

Summary:
Ref T13042. This adds a "silent" edit mechanism which suppresses feed stories, email, and notifications.

The other behaviors here are:

- The transactions are marked as "silent" so we can render a hint in the UI in the future to make it clear to users that they aren't missing email.
- If the editor uses Herald, mail rules are suppressed so they don't fire incorrectly (this mostly affects "the first time this rule matches, send…