* RepoBranch now has a `can_push` field indicating whether the API
  user can push to the branch.

Fetch api v4 sample data from a GitLab 9.2 instance.  Using that and
GitLab sources `doc/api/v3_to_v4.md` and `lib/api/entities.rb`, update
our types accordingly.  Most changes are to rename "builds" to "jobs" or
"pipelines".

Also add strong types to internal ids in preparation for switching to
them when constructing API endpoints.