mirrors: implement refs namespace cleaning
Some hosting providers like to include all kinds of "extra" refs in the
refs namespace. Things the project contributors never pushed and
generally are not allowed to push to.
Therefore provide a refs namespace "cleaning" service for mirrors which
is selected by default for new mirror project registrations and when
enabled causes the mirror to only mirror refs/heads/*, refs/tag/* and
refs/notes/* refs thereby "cleaning" the mirror.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>