move "fetching" log msg after validation checks
drop FB and instagram poll frequency from 5m to 10m
delete out of date comments
start to implement publish/listen distinction

also fix bug introduced in 38ccdc87b383cb35465e67a2ce7becc204ab2e63: actually overwrite source entity when updating.
merge features field when updating source
start to implement publish page and separate listen/publish signup and oauth scopes
unify add and delete oauth callback handlers into Handler.maybe_add_or_delete_source()
only add new poll tasks for listen sources