Add patch tag infrastructure

This change add patch 'tags', eg 'Acked-by' / 'Reviewed-by', etc., to
patchwork.

Tag parsing is implemented in the patch parser's extract_tags function,
which returns a Counter object of the tags in a comment. These are
stored in the PatchTag (keyed to Tag) objects associated with each
patch.

We need to ensure that the main patch lists do not cause per-patch
queries on the Patch.tags ManyToManyField (this would result in ~500
queries per page), so we introduce a new QuerySet (and Manager) for
Patch, adding a with_tag_counts() method to populate the tag counts in a
single query.

As users may be migrating from previous patchwork versions (ie, with no
tag counts in the database), we add a 'retag' management command.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>