Display a banner when XWiki is in readonly mode

Goal: Let users of the user understand that the wiki is in read only mode / maintenance and that they cannot make changes to the wiki or register.

Several points:

• I think this shouldn't be implemented in a wiki page since you can set readonly with an empty wiki. So it should be implemented in a template.
• It would be nice to offer a config property and an Admin UI to be able to configure the message to display.