REST: Fix duplicate project queries

REST: Fix duplicate project queries

Eliminates duplicate project queries caused by calling
get_absolute_url() in the embedded serializers. Following foreign keys
with 'series__project' will cache the project of the series as well as
the series itself.

Signed-off-by: Mete Polat <metepolat2000@gmail.com>
Signed-off-by: Stephen Finucane <stephen@that.guru>
Closes: #335
(backported from commit ec00daae4d79bf2560034e1b2bc3cf76a98a3212
  dropped all the tests, they clash horribly)
Signed-off-by: Daniel Axtens <dja@axtens.net>