Simplify ownership for Doorkeeper::Application

Remove whitelist for owners ... revert back to single owner
Remove Memberships and many-to-many w/through
Keep developer_emails whitelist

Result is that Applications have one owner, which is a one-to-many relation and if more people need to use a private app, they can be added as developers.
Require current user to be owner to update application
remove commented out code
Merge pull request #379 from 18F/hotfix/simplify_app_ownership

Hotfix/simplify app ownership
Merge branch 'devel' into feature/openid-connect-2