drop DB rules
I applied branch.sql to production; #2472.

It was a data update, so no need to append to schema.sql.
Merge branch 'master' into drop-db-rules

Conflicts:
	gittip/models/participant.py
Merge pull request #2358 from gittip/drop-db-rules

Drop remaining DB rules
Bump version to 13.1.3
Bump version to 13.1.3-dev
Append branch.sql to schema.sql

Only including schema-relevant bits, not data changes.
Implement clear_tips_giving

Method needed as part of account closing, #2412.