This commit fixes handling of branches when using both V3 and V2. The
changes include:
* proper definition of relationships that reflect V3 structure, so for
example build belongs to a branch
* setting up inverse records for some of the relationships. without
doing that Ember Data can handle relationships in a surprising way,
for example if the same record is referenced in 2 places in a
belongsTo relationship, Ember Data will remove one of the references
without proper inverse definitions
* we need to add id when extracting branch as a relationship. Ember
Data expects all of the relationships to have an id
* lastly, we need to mimic the structure of the V3 API in V2 payloads,
so for a build payload I'm now creating a branch record
* we should look for both embedded relationship and relationship key,
so in cases like for commit, when there's a full commit data on
"commit" property, and only id at "commit_id", we will use commit data
* we can't add @type to V2 fallback, because in other places we chack
for @type to distinguish V2 and V3 payloads
* there's no need to include a record in "included" if there's only a
type and an id there
This commit adds adapters and serializers for v3, but also a fallback
serializer for v2, which allows to handle v2 and v3 payloads at the same
time. This is needed, because when we use v3 endpoint for one of the
models (in this case repo), we can also get embedded records of other
types (like branch or build).
