REST: Null out previous, current relation info

REST: Null out previous, current relation info

These fields don't work like we expect them to. Because we're linking to
a non-idempotent entity, an instance of 'relation', what we're storing
in either of these fields is subject to change as patches are added and
removed. This makes the information pretty much useless after the fact.
It's best to just state the patch and request that people query the
information themselves if necessary. We don't want to remove the field
entirely from the API - that would be a truly breaking change - so
instead we null it out like we do for patch tags. In a v2 API (i.e. a
major version bump) we can remove this entirely.

A small bug with the schema generation is corrected.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Related: #379
(cherry picked from commit 646a2f2c80056a33c70efd760446832f90899247)