Read-only mode is a setting an admin can enable to prevent writes to

the database. This can be used when the site is under maintenence or

being upgraded. The previous commit added the siteconfig attribute and

a checkbox to toggle the mode. This commit adds a new 503 error to

Review Board's webapi. Furthermore, all requests that hit RB's API

using PUT, POST, DELETE methods while read-only mode is on, and

the user is not a superuser, will return the new 503 error. A utility

file read_only.py has been added which deals with the logic and

caching of determining when a user should be affected by read-only

mode.

Added new Python unit tests that send API requests as superuser and

non-superuser

Ran the Python unit test suite

Check that modifying/deleting review requests and comments works

properly