This adds a new model entity `DesignManagement::DesignAtVersion`, which
wraps a design and an associated version. Mechanisms are provided for
lazy-finding and validation.

These do not have mutable properties, and validate the important
constraint that they share an issue

Finding DesignManagement::DesignAtVersion objects requires querying both
the designs and the versions tables, and is at present more expensive
than a single fetch. It also returns an Array instead of a relation,
since this is not a ActiveModel object. We validate invariants in find,
this ensures we never fetch invalid objects.

* Validations:
  - checking that there is both a design and a version
  - checking that the design and the version have the same issue
  - checking that both the design and the version have an issue

  These validations are checked during `find` and when calling
  `instantiate`

* ID

  It is important for GraphQL and the front-end that the model has an ID
  that can be used to lookup each combination, and be used to store this
  identity on the client.

* deleted? and status methods

  These supplement their analogues in DesignManagement::Design with an
  awareness of the current version. Thus a
  DesignManagement::DesignAtVersion as of a version in which is was
  deleted will respond to `deleted?` appropriately.

  Callers should be be careful to distinguish between `dav.state ==
  :current` and `!dav.deleted?` - as of a version before a design has
  been created it will respond with:
   * `#deleted? == false`
   * `#status == :not_created_yet`
  Reflecting the fact that this is not deleted because it has not been
  created yet.

This requires changes to the design and version factories to be able to
create valid objects with nil issues, a condition that is found when
these objects are being imported.