Read-only mode is a feature that can be enabled by the admin to disable

most backend changes by regular users. This commit adds a custom

message for the banner when in read-only mode. The message can be

optionally added through the admin panel right under where the admin

would enable/disable read-only mode.

Ran js-tests - passed

Ran python tests - most passed, some failures fixed in future commit

Checked changing value in admin panel reflects in banner