Bug 1074851 - save job exclusions as a flatlist of signatures

 - saves exclusions as a list of refdata signatures
 - the get_resultset_jobs endpoint honors the default exclusion profile via the signatures
 - group exclusion signatures by repo for performance

Also added handling for a profile with no job exclusions
Merge pull request #292 from mozilla/server-hidden-jobs

Bug 1074851 - Implement server side job filtering