Conditional release

Adds feature to allow instructors to restrict release of a set (homework or gateway) based on percentage score(s) on other designated set(s).  

Adds two fields to the global set and user set table: restricted_release and restricted_status.  

Adds two corresponding fields to ProblemSetDetail.pm.  In the "Restrict release by set(s):" field, the instructor can enter a comma separated list of homework sets.  

In the "Status required for release:" field the instructor specifies a number between 0 and 1 that will be the percentage score students must achieve on all sets listed in the previous field to release this set.  Students will see appropriate messages in the homework sets list.  

Error handling:  If an instructor enters an invalid set in the "Restrict release by set(s)" field, webwork will warn on save and will not attempt to use this set as a restriction.