allow configuring designated registration org to which new users can register

[ikreymer]

if 'registration_enabled' is set, check 'registration_org_id' for org id of an existing org that new users should be added to when they register. if omitted, default to the default org

Fixes #1729

Test instructions:

1. Create a new org
2. set 'registration_org_id' in local chart to new org's id
3. set 'registration_enabled: 1'
4. Register as new user
5. Ensure new user is added to the designated registration org, not the default org

[tw4l]

Thanks! Tested