Arvados model now validates that all "foreign_key_attributes" exist before create/update.
Fighting with tests
All tests pass!
Added 'is_a' filter and tests, refs #2418 #2228
Removed internal uses of _kind column in API server.  Tests pass.
Actually remove _kind columns from schema and tests.
Removed '_kind' from documentation.
Removed head_kind and tail_kind from workbench.
Removed some trailing whitespace.
Merged master
API server tests pass, workbench tests are failing
Merge remote-tracking branch 'origin/master' into origin-2228-check-filter-uuid-columns

Conflicts:
	services/api/app/models/arvados_model.rb
* Refactored to remove load_kind_params filter and instead override load_where_param and load_filters_param in the links and logs controllers to add the _kind functionality.
* Removed 'rescue' clause.  I think I left that in from debugging.
* Fixed :log -> :logs
* filters: ['tail_kind','=','arvados#user'] works
* Added more tests.
* Added equivalent logic and tests for logs controller for object_kind
Merge remote-tracking branch 'origin/master' into origin-2228-check-filter-uuid-columns
Reverted accidental change to Gemfile.
Restored parameter check in load_filters_param.
Removed spurious commented-out code from test.
Links controller checks that head/tail_kind matches head/tail_uuid.
Added test.
Reverted Gemfile.lock