REST: Make 'Patch.state' editable

REST: Make 'Patch.state' editable

The 'Patch.state' field is exposed by the API, but is not editable.
Tests exist that suggest the field is editable, but they lie as they
don't actually validate the field is changed (it hasn't). Make this
field editable, using a custom field type to allow said user to submit a
string representation of the state rather than an integer id.

This has the side effect of changing the output representation of the
'state' field for the '/patches' endpoint to a slugified representation,
i.e.:

    "state": "under-review",

Instead of:

    "state": "Under review",

The same slugified representation will be used in PATCH requests. This
seems more consistent with how APIs generally do this stuff making it a
"good thing" (TM).

Signed-off-by: Stephen Finucane <stephen@that.guru>
Reviewed-by: Andy Doan <andy.doan@linaro.org>
Reviewed-by: Daniel Axtens <dja@axtens.net>