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 me an email" rules: without this, they'd match "the first time" on the bulk edit, not send email, then never match again since they already matched).
• If the edit queues additional edits, those are applied silently too.

This doesn't (or, at least, shouldn't) actually change any behavior since you can't apply silent edits yet.

Test Plan:
Somewhat theoretical, since this isn't reachable yet. Should get meaningful testing in an upcoming change.

Did a bit of var_dump() / debug poking to attempt to verify that nothing too crazy is happening.

Viewed and edited objects, no changes in behavior.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13042

Differential Revision: https://secure.phabricator.com/D18882