REST: filter patches by state name

REST: filter patches by state name

Since we display the name of the state in the serialized patch, also
filter by name instead of primary key.

Also this will be consistent with the current documentation examples
(curl 'https://patchwork.example.com/api/patches?state=under-review')

Signed-off-by: Philippe Pepiot <philippe.pepiot@logilab.fr>
Reviewed-by: Stephen Finucane <stephen@that.guru>