Add a cluster.read-only option

Summary:
Ref T4571. There will be a very long path beyond this, but add a basic read-only mode. You can explicitly enable this to put Phabricator in a sort of "maintenance" mode today if you're swapping databases or something.

In the long term, we'll automatically degrade into this mode if the master database is down.

Test Plan:

• Enabled read-only mode.
• Browsed around.
• Didn't immediately see anything that was totally 100% broken.

Most stuff is 80-90% broken right now. For example:

• Stuff like submitting comments doesn't work, and gives you a confusing, unhelpful error.
• None of the UI really knows that it's read-only. EditEngine stuff should all hide itself and say "you can't add new comments while an install is in read-only mode", for example, but currently does not.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4571

Differential Revision: https://secure.phabricator.com/D15662