Add an is_closed attribute to Participant

This is useful for showing an "Account closed by user" page for that
case.
set is_closed=False in opt_in() instead of in set_as_claimed()
Merge pull request #2485 from gittip/is_closed

Add an is_closed attribute to Participant
merge branch.sql into schema.sql
Bump version to 13.1.6
Bump version to 13.1.6-dev
cache giving, pledging and receiving amounts in the participants table
remove Participant.get_dollars_* functions
small refactoring
rewrite queries to use the cached amounts
fix bad uses of set_attributes()
don't query the DB when we already have the Participant
add a few tests
Bump version to 13.1.7
Bump version to 13.1.7-dev
Merge pull request #2352 from gittip/cache-giving-receiving

Cache giving, pledging and receiving amounts
merge branch.sql into schema.sql
Bump version to 13.1.8
Bump version to 13.1.8-dev
don't archive participant when closing account

but set claimed_time to NULL