REST: Add patch relations

View relations and add/update/delete them as a maintainer. Maintainers
can only create relations of patches which are part of a project they
maintain. Because this is a writable many-many nested relationship, it
behaves a little unusually. In short:

- All operations use PATCH to the 'related' field of a patch

- To relate a patch to another patch, say 7 to 19, either:

    PATCH /api/patch/7  related := [19]
    PATCH /api/patch/19 related := [7]

- To delete a patch from a relation, say 1, 21 and 42 are related but we
  only want it to be 1 and 42:

    PATCH /api/patch/21 related := []

  * You _cannot_ remove a patch from a relation by patching another
    patch in the relation: I'm trying to avoid read-modify-write loops.

  * Relations that would be left with just 1 patch are deleted. This is
    only ensured in the API - the admin interface will let you do this.

- Break-before-make: if you have [1, 12, 24] and [7, 15, 42] and you want
  to end up with [1, 12, 15, 42], you have to remove 15 from the second
  relation first:

    PATCH /api/patch/1 related := [15] will fail with 409 Conflict.

  Instead do:

    PATCH /api/patch/15 related := []
    PATCH /api/patch/1  related := [15]
       -> 200 OK, [1, 12, 15, 42] and [7, 42] are the resulting relations

Signed-off-by: Mete Polat <metepolat2000@gmail.com>
Signed-off-by: Stephen Finucane <stephen@that.guru>
Signed-off-by: Daniel Axtens <dja@axtens.net>

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