I've been postponing this for some time, but it's better to have a
foreign key to the latest series than caching some of its data on the
Series object.

We still want that link on Series, because retrieving the data held on
the revision is a simple JOIN, so we can still have our O(1) number of
SQL queries when retrieving data for the series list page.

While at it and to limit the number of migration steps, we move the
n_patches to SeriesRevision, where it should have been in the first
place. Also the version field can simply be the one on last_revision
now.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>