[SCHEMA] Rename account.default_calendar_id foreign key constraint.

Summary:
This is an attempt to work around this bug:
http://bugs.mysql.com/bug.php?id=70171
in MySQL versions 5.6.12 and 5.6.13, which was causing migration 083 to fail
when run against RDS.

Test Plan:
Run migrations both on a local VM previously synced to head, and on
gunks.

Reviewers: charles

Reviewed By: charles

Differential Revision: https://review.inboxapp.com/D388