REST: Allow setting of values using embedded serializers

REST: Allow setting of values using embedded serializers

Unfortunately, the use of embedded serializers for some fields breaks
the ability to update these fields, either via the HTML interface (where
the widget is totally busted) or via a client like 'git-pw'. What we
actually want is to be able to update these fields like normal primary
key but show them using the embedded serializer. We do just this by
using a modified variant of the PrimaryKeyRelatedField and using the
serializers simply for displaying.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Closes: #216