ENH: Revamp mmm fixture import code

- Old fuzzy matching code was throwing away ~75% of the matches
  included in the file. expunged.
- Now relies on entity_id field already included in the fixture,
  providing 6x more matches, relying on a faster and more accurate
  algo.
- Much faster import process with @transaction.commit_manually
- Uses a single JSON fixture file.
- Added retrieval_date and schema_version to fixture file.
- Data lives at:
	https://github.com/y-p/mmm_scrape/blob/master/results/mmm.json
  occasionally updated.
- Moved test fixture file to "testdata/mmm".
- Placed gzipped fixture in "data/".
- Reworked tests to accommodate changes. less moving parts now.
- Closes the long-ignored #3303.
Merge branch 'feature/mmm_import_improvements' of https://github.com/y-p/Open-Knesset