Add siteconfig var and button for read-only mode

Review Request #8648 — Created Jan. 20, 2017 and submitted — Latest diff uploaded

Information

khp
Review Board
master
7ffda32...

Reviewers

Read-only mode is a new feature that allows users to view Review
Board while the site is being maintained or upgraded.
Added a toggle for read-only mode in Admin -> General Settings by
adding a checkbox.
Added a new attribute to siteconfig called site_read_only.
Toggling read-only mode will toggle the attribute to true/false.

In admin panel set new toggle to on/off and check that site_read_only
in siteconfig is set to true or false depending on whether the toggle
is checked or not.