When Phabricator is in read-only mode, explain why

Summary:
Ref T4571. Allows users to click the "read-only mode" notification to get more information about why an install is in read-only mode.

Installs can be in this mode for several reasons (explicit administrative action, no masters defined, no masters reachable), and it's useful to be able to tell the difference.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4571

Differential Revision: https://secure.phabricator.com/D15671