Provide support for defining more granular authorizations for our future Perf sheriffs. Presumably, they'll share various skills & experience levels so they can't be simply granted access to everything on Perfherder/TaskCluster.

For start, just implement 2 ranks:

• community level rank, with very low access (barely more than public read access; will definitely lack retrigger/backfill on TaskCluster, as that requires Commit Access Level 3)
• full access rank, similar to what me/bebe/jmaher/davehunt etc already have

I don't think we have any benefits for this, at least not for the upcoming 2-3 quarters. Will reopen if things change.