CDB_CartodbfyTable: enforce quota triggers (#169)

Has rails install a _CDB_UserQuotaInBytes() function to advertise
user setting so that quota triggers do can call into that function
to know what is or isn't allowed.

Tweak some rake tasks related to triggers to basically end up calling
the cartodbfier function
Mark _CDB_UserQuotaInBytes as IMMUTABLE, to allow for caching

The function will be re-created on quota change anyway