A wiki can be made readonly like so:
$wgReadOnly = true;
When true the edit icon should appear as locked and when clicked should display the toast message "apierror-readonly": "The wiki is currently in read-only mode.",
Make sure T197499 is also on the sprint board! You'll want to do that task in close proximity to this one.
== acceptance criteria
 Use same icon as the protected icon when wiki is in read only mode
== qa steps
* Make sure you are an anonymous user and on a page with an edit icon that is clickable.
* Ask an engineer to set staging to read only mode
* Verify that after doing so, the edit icon displays as locked
* Ask an engineer to set staging to write mode
* Verify on same page the edit icon displays as unlocked