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

endpoints/api/user.py

@@ -306,6 +306,7 @@ class User(ApiResource):
     return user_view(user)
 
   @show_if(features.USER_CREATION)
+  @show_if(features.DIRECT_LOGIN)
   @nickname('createNewUser')
   @internal_only
   @validate_json_request('NewUser')
@@ -496,6 +497,7 @@ class ConvertToOrganization(ApiResource):
 
 
 @resource('/v1/signin')
+@show_if(features.DIRECT_LOGIN)
 @internal_only
 class Signin(ApiResource):
   """ Operations for signing in the user. """
@@ -595,6 +597,7 @@ class Signout(ApiResource):
 
 
 @resource('/v1/detachexternal/<servicename>')
+@show_if(features.DIRECT_LOGIN)
 @internal_only
 class DetachExternal(ApiResource):
   """ Resource for detaching an external login. """