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

static/partials/user-view.html

@@ -43,7 +43,8 @@
               tab-init="showInvoices()" quay-show="Features.BILLING">
           <i class="fa ci-invoice"></i>
         </span>
-        <span class="cor-tab" tab-title="External Logins" tab-target="#external">
+        <span class="cor-tab" tab-title="External Logins" tab-target="#external"
+              quay-show="!hasSingleSignin">
           <i class="fa fa-external-link-square"></i>
         </span>
         <span class="cor-tab" tab-title="Authorized Applications" tab-target="#applications"
@@ -70,7 +71,7 @@
         </div>
 
         <!-- External Logins -->
-        <div id="external" class="tab-pane">
+        <div id="external" class="tab-pane" quay-show="!hasSingleSignin">
           <div class="external-logins-manager" user="viewuser"></div>
         </div>