Remove 'PatchRelationSerializer'

Remove 'PatchRelationSerializer'

This wasn't writeable for reasons I haven't been able to figure out.
However, it's not actually needed: the 'PatchSerializer' can do the job
just fine, given enough information. This exposes a bug in DRF, which
has been reported upstream [1]. While we wait for that fix, or some
variant of it, to be merged, we must monkey patch the library.

Conflicts:
	patchwork/api/patch.py

NOTE(stephenfin): Conflicts are due to the absence of commit d3d4f9f36
("Add Django 3.0 support") which we do not want to backport here.

[1] https://github.com/encode/django-rest-framework/issues/7550
[2] https://github.com/encode/django-rest-framework/pull/7574

Signed-off-by: Stephen Finucane <stephen@that.guru>
Reported-by: Ralf Ramsauer <ralf.ramsauer@oth-regensburg.de>
Closes: #379
Cc: Daniel Axtens <dja@axtens.net>
Cc: Rohit Sarkar <rohitsarkar5398@gmail.com>
(cherry picked from commit fe07f30df6fe263938b1f898ffffc354c4ff470c)