Add support for custom "Wait for Approval" instructions

Summary:
See PHI1229. An install has a somewhat duct-taped registration flow which can dump users on the "Wait for Approval" screen without clear guidance. The desired guidance is something like "this is totally normal, just wait a bit for a bot to approve you".

Adding guidance here is generally reasonable and consistent with the intent of this feature.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: kylec

Differential Revision: https://secure.phabricator.com/D20492