Allow schema properties to override mixin properties.
Reorganise experiment so dataset is a base type.
Rejig json rendering so that links are filled before templates; upgraded properties are used for links; simplify rev_links to same rules as add_dependent_objects.add_dependent_objects.
File sample data; more fixtures.
dataset/experiment.files is now a combination of additional_files and reverse file links.
dataset/experiment upgrades.
Merge branch 'master' into experiment-files-650
Add experiment and dataset to list for upgrading.
Handle embedding template values in create_mapping; rejig merging of class dict attributes; make Experiment inherit from dataset more cleanly.
Merge branch 'fix-test-data-line-endings' into experiment-files-650

Conflicts:
	src/encoded/tests/data/inserts/dataset.tsv
	src/encoded/tests/data/inserts/experiment.tsv
Remove additional_files which are already homed in the experiment.
Hide DELETED and OBSOLETE status items from reverse links.
Update test_edw_sync to account for extra file line.
Merge branch 'experiment-files-650'
Give dummy request the _stats dict. The stats events are setup to listen on all Engines.
Use subprocess postgres for tests by default (avoid sqlite segfault.)
Fix transaction record row insert after a savepoint rollback.
Fix transaction recording.
Fix sync_edw.get_all_datasets to use correct loop variable.

The tests here are really brittle and could really do with reworking sometime.
Merge branch 'master' into GO_ids-940