assignsubmission_comments: Improve performance for permissions checking

When viewing the grading table, a permissions callback will check for suspended users for each submission in the table.

Please view the attached callgraph alongside my explanation regarding my changes.

In assign_submission_comments::view_summary(), there is a call to the comment constructor. This constructor calls comment::check_permissions(), which is expensive – for each row!

According to the callgraph, the reason why comment::check_permissions() is expensive, is not because it calls has_capability(), but rather because it calls assignsubmission_comments_comment_permissions().

If you view assignsubmission_comments_comment_permissions(), it checks get_suspended_userids(), which generates an expensive SQL query in pg_query_params().

My fix is to cache the result of get_suspended_userids() in a MODE_REQUEST Cache.