models: Merge 'Patch' and 'Submission'

models: Merge 'Patch' and 'Submission'

Oh, the follies of youth. Time to undo the damage of 2.0.0, specifically
commit 86172ccc16, which split Patch into two separate models using
concrete inheritance. As noted previously, this introduced a large
number of unavoidable JOINs across large tables and the performance
impacts these introduce are blocking other features we want, such as
improved tagging functionality. To combine these two models, we must do
the following:

- Update any references to the 'Patch' model to point to the
  'Submission' model instead
- Move everything from 'Patch' to 'Submission', including both fields
  and options
- Delete the 'Patch' model
- Rename the 'Submission' model to 'Patch'

With this change, our model "hierarchy" goes from:

  Submission
    Patch
    PatchComment
  Cover
    CoverComment

To a nice, flat:

  Patch
    PatchComment
  Cover
    CoverComment

I expect this migration to be intensive, particularly for MySQL users
who will see their entire tables rewritten. Unfortunately I don't see
any way to resolve this in an easier manner.

Signed-off-by: Stephen Finucane <stephen@that.guru>