Add read-only banner under navbar

Review Request #8812 — Created March 11, 2017 and submitted — Latest diff uploaded


Review Board


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. This commit adds a banner under the nav bar when the
site is in read-only mode.

Check that the banner appears in read-only mode
Check that the banner does not appear when not in read-only mode