models, templates: Add patch relations

models, templates: Add patch relations

Introduces the ability to add relations between patches. Relations are
displayed in the details page of a patch under 'Related'. Related
patches located in another projects can be viewed as well.

Changes to relations are tracked in events. Currently the display of
this is very bare in the API but that will be fixed in a subsequent patch:
this is the minimum required to avoid throwing errors when you view the
events feed.

Signed-off-by: Mete Polat <metepolat2000@gmail.com>
[dja: address some review comments from Stephen, add an admin view,
      move to using Events, misc tidy-ups.]
Signed-off-by: Daniel Axtens <dja@axtens.net>