Add batch lookup for user groups on Special:ListUsers

At the moment for each user on the list a new user object is created and
than the getGroups method is called, which fully init the user object,
which is done in 3 queries (user table, user_properties and
user_groups).
Removed the user object with one query for all the userids and cache the
result.

Also added the group page to the LinkBatch, because now the necessary
pages can be determined easily.

Change-Id: I4a945f83ad28edf5cc040139943cf743cb3d133c
Merge "Add batch lookup for user groups on Special:ListUsers"