user/index: Show enrolment and group/grouping data in Participants page MDL-12311

user/index: Show enrolment and group/grouping data in Participants page MDL-12311

The participants page will now show (to users that have
moodle/role:assign) the relevant enrolments for each participant.

This is done with a second SELECT using a WHERE id IN() so we are
limited to doing it with small datasets. This is for 2 reasons

 - we cannot retrieve all teh data in the "main" SELECT because
   the outer joins will bring dup rows, breaking LIMIT

 - we cannot use the main SELECT as a subselect because we cannot
   retrieve the LIMIT part of the statement with the current dmllib

 - on large courses / sitecourse performance would be horrible

so it is limited to paginated datasets. It is doing 20 recs at a time,
the "extra" SELECT takes ~7ms on a some large Pg databases).

This will generate an extra DB query on Oracle (and any other DB that
cannot rewind the recordset) because we walk the main rs twice.

While at it, ensure we mark the unsortable columns thus...