MDL-21933 course: Allow users to search participants course permission

Capability checks are performed at the top of the page and do not need
to be completed again.

Search is an expected feature for users and it was confusion to only
appear after 3 pages.  This has been changed to enable search when
there are more records than are visible on the screen.  If users
are unable to use browser search, they will be able to use Moodle's
search.