Add proper and tested OIDC support on the server

Note that this will still not work on the client side; the followup CL for the client side is right after this one.

test/testconfig.py

@@ -81,11 +81,12 @@ class TestConfig(DefaultConfig):
 
   FEATURE_GITHUB_LOGIN = True
   FEATURE_GOOGLE_LOGIN = True
-  FEATURE_DEX_LOGIN = True
 
-  DEX_LOGIN_CONFIG = {
-    'CLIENT_ID': 'someclientid',
-    'OIDC_SERVER': 'https://oidcserver/',
+  TESTOIDC_LOGIN_CONFIG = {
+    'CLIENT_ID': 'foo',
+    'CLIENT_SECRET': 'bar',
+    'OIDC_SERVER': 'http://fakeoidc',
+    'DEBUGGING': True,
   }
 
   RECAPTCHA_SITE_KEY = 'somekey'