Add rough validation on email addresses

Summary: Put a very rough filter on what we'll accept as an email address. We can expand this if anyone is actually using local delivery or other weird things. This is mostly to avoid a theoretical case where some input is parsed differently by `PhutilAddressParser` and the actual mail adapter, in some subtle hypothetical way. This should give us only "reasonable" email addresses which parsers would be hard-pressed to trip up on.

Test Plan: Added and executed unit tests. Tried to add silly emails. Added valid emails.

Reviewers: btrahan, arice

Reviewed By: arice

CC: arice, chad, aran

Differential Revision: https://secure.phabricator.com/D8320