Add support for Dex to Quay

Fixes #306

- Adds support for Dex as an OAuth external login provider
- Adds support for OIDC in general
- Extract out external logins on the JS side into a service
- Add a feature flag for disabling direct login
- Add support for directing to the single external login service
- Does *not* yet support the config in the superuser tool

initdb.py

@@ -218,6 +218,7 @@ def initialize_database():
   LoginService.create(name='ldap')
   LoginService.create(name='jwtauthn')
   LoginService.create(name='keystone')
+  LoginService.create(name='dex')
 
   BuildTriggerService.create(name='github')
   BuildTriggerService.create(name='custom-git')