Allow user to upload custom operating systems

[alexchristy]

• This would probably happen after we implement user authentication, as we don't want to deal with licensing issues

• The database can store AMIs/URNs for an OS as well as the minimum disk space, etc.

• Maybe when this happens, we query from the DB instead of from python? That way we can load the enums into the DB and easily add to it over time.

• User query for AMIs would just be a join of the community AMIs and the ones they uploaded.

From @Adamkadaban at #34