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.

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.