started some Transitions API docs. WIP medic/medic-webapp#714
refactored changes feeds WIP medic/medic-webapp#741

removed any server side filters.

changed `since` value on transitions feed to subtract 50 from update_seq
on start up to catch any changes between a restart.

added `db_info` export on config module.

modified `update_sent_forms` transition to check for a config that has a
form property before running. so only runs when config entry matches.