(bug 62491) don't use EntityContent in API modules

Change-Id: I886644a93a713fca3bec68e9a14549a846686bbf
(bug 62491) Service for checking permissions.

This introduces the EntityPermissionChecker service and
removes any methods for permission checking from EntityContent.
That allows us to get rid of the last reason ApiWikibase
needed to depend on EntityContent.

Change-Id: I3323ba922e33171563ed69ffa64a64829b563ae1
Merge "(bug 62491) don't use EntityContent in API modules"
Merge "(bug 62491) Service for checking permissions."