models: Split Patch into two models

models: Split Patch into two models

There are a lot of similarities between cover letters and patches: so
many, in fact, that it would be helpful to occasionally treat them as
the same thing. Achieve this by extracting out the fields that would be
shared between a Patch and a hypothetical cover letter into a "sub
model". This allows us to do cool stuff like assigning comments to both
patches and cover letters or listing both patches and cover letters on
the main screen in a natural way.

The migrations for this are really the only complicated part. There are
three, broken up into schema and data migrations per Django customs,
and they works as follows:

* Rename the 'Patch' model to 'Submission', then create a subclass
  called 'Patch' that includes duplicates of the patch-specific fields
  of Submission (with changed names to prevent conflicts). Rename
  non-patch specific references to the renamed 'Submission' model
  as necessary.
* Duplicate the contents of the patch-specific fields from 'Submission'
  to 'Patch'
* Remove the patch-specific fields from 'Submission', renaming the
  'Patch' model to take their place. Update the patch-specific
  references to point the new 'Patch' model, rather than 'Submission'.

This comes at the cost of an additional JOIN per item on the main
screen, but this seems a small price to pay for the additional
functionality gained. To minimise this, however, caching will be added.

Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Signed-off-by: Andy Doan <andy.doan@linaro.org>