Allow MFA enrollment guidance to be customized

Summary: Depends on D20039. Ref T13242. If installs want users to install a specific application, reference particular help, etc., let them customize the MFA enrollment message so they can make it say "if you have issues, see this walkthrough on the corporate wiki" or whatever.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13242

Differential Revision: https://secure.phabricator.com/D20043