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

config.py

@@ -153,6 +153,9 @@ class DefaultConfig(object):
   # Feature Flag: Whether Google login is supported.
   FEATURE_GOOGLE_LOGIN = False
 
+  # Feature Flag: Whther Dex login is supported.
+  FEATURE_DEX_LOGIN = False
+
   # Feature flag, whether to enable olark chat
   FEATURE_OLARK_CHAT = False
 
@@ -184,6 +187,9 @@ class DefaultConfig(object):
   # Feature Flag: Whether to automatically replicate between storage engines.
   FEATURE_STORAGE_REPLICATION = False
 
+  # Feature Flag: Whether users can directly login to the UI.
+  FEATURE_DIRECT_LOGIN = True
+
   BUILD_MANAGER = ('enterprise', {})
 
   DISTRIBUTED_STORAGE_CONFIG = {