[Custom User Groups] Allow custom user group membership to be updated via SIS Import

[eric_silva]

At our institution, we try to use custom user groups as much as possible to target our messaging. Sometimes the population is static, but they're often dynamic as we're targeting all online students or only students who haven't submitted their course evaluations. Rather than having to update these files manually and sometimes daily, it would be easier if we could drop a daily file via SFTP and have multiple custom user groups updated. 

[MarcoMoreno]

Great idea!