Implement local user accounts

[philipashlock]

Currently Github is used for all user accounts. We should both allow for other OAuth providers as well as autonomous local users. This also means usernames in DataBeam would be independent of usernames in Github or other third party services. In a similar vein, it could make sense for there to be a configuration option that makes an instance single-user instead of multi-user - mostly this would be apparent in the URLs which currently incorporate usernames.