Only add students to Special:Students

Only add students, and not other roles, to the Students table and
the list of students on Special:Student. This only applies to new
user particpations in courses.

bug: 44110
Change-Id: I2d76d44862e6f82996f1c9140b7a48a93c169803
Fix enrollment status on Special:Student

Instead of using the unreliable enrollment status value stored in
the ep_students table, we query the database on every page view.
It's only one query.

Change-Id: Ic201725f2f01a0322e52baf04c2b9c0483189b5d
Merge "Only add students to Special:Students"
Merge "Fix enrollment status on Special:Student"