Add a placeholder for status of datacenter read-only mode

A while ago Confluence introduced the read-only mode which ensures that no data is altered while it is active:

https://developer.atlassian.com/server/confluence/how-to-make-your-add-on-compatible-with-read-only-mode/

We need a way to know in a viewport if the read-only mode is enabled so that we disable data altering actions like providing feedback.

Therefore we need a placeholder which tells the theme if the read-only mode is enabled or not.