Introduce new userrole xm-content-admin and rename xm-content-reader to xm-content-viewer

Through the xm-content-admin userrole functionalities like 'unlock a document' can be granted by default to a specific user/group without needing to be 'xm-admin'

Also, rename xm-content-reader to xm-content-viewer to better align with xm-channel-viewer. 
Furthermore, in the future we also may  need a (separated) xm-content-reviewer, for which this renaming would (then) become needed.