Don't publish anything for "Disable Mail" / "Enable Mail" on projects

Summary:
Fixes T10466. Currently, clicking "Disable Mail" or "Enable Mail" on a project toggles an edge, but it gets a default "added an edge" story and transaction record.

These are confusing, useless and not interesting, so just hide them.

Test Plan:

• Before patch: clicked enable/disable mail, saw "added an edge" / "removed an edge" stories in feed and project history.
• After patch: clicked enable/disable mail, saw nothing in feed or project history.
• (Note that this patch is not retroactive for feed, so already-published stories won't unpublish.)

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10466

Differential Revision: https://secure.phabricator.com/D15361