Add support for temp usernames and an interstitial to confirm username

When a user now logs in for the first time for any external auth (LDAP, JWT, Keystone, Github, Google, Dex), they will be presented with a confirmation screen that affords them the opportunity to change their Quay-assigned username.

Addresses most of the user issues around #74

static/directives/namespace-input.html

@@ -1,5 +1,7 @@
 <span class="namespace-input-element">
   <input type="text" class="form-control" placeholder="{{ namespaceTitle }}" ng-model="binding"
          required autofocus ng-pattern="usernamePattern"
-         name="namespaceField">
+         ng-model-options="{'debounce': 200}"
+         name="namespaceField"
+         ng-class="hasExternalError ? 'ng-invalid' : ''">
 </span>