WR498018 Fix grade performance

[SimonThornett]

Fixes #118

I've made the following changes in mod/subcourse/locallib.php:

• New helper subcourse_get_gradeable_userids():

  • When user IDs are passed: it checks each one with has_capability('mod/subcourse:begraded', $context, $userid, false), so the admin override is ignored as before. Duplicates, zero or negative IDs and the guest user are skipped, and deleted users fail the check. This is the path the event observers and lib.php:465 use, so a single-user event no longer loads the course roster.
  • When no user IDs are passed: one query using get_with_capability_join(), returning user IDs only and no longer sorting by lastname.

• Grades are now loaded in one batch with grade_grade::fetch_users_grades(), instead of one query per user. How each grade is worked out hasn't changed.

Tests: I added test_subcourse_fetch_refgrades_requested_users to the existing tests/locallib_test.php. It covers passing a single user, users without the capability, the admin and guest users, invalid IDs, a user with no grade, and fetching everyone.