Add feature flag to disable username confirmation

Fixes https://jira.coreos.com/browse/QUAY-914

static/directives/config/config-setup-tool.html

@@ -1350,6 +1350,18 @@
               </div>
             </td>
           </tr>
+          <tr>
+            <td class="non-input">Allow username confirmation:</td>
+            <td colspan="2">
+                <div class="config-bool-field" binding="config.FEATURE_USERNAME_CONFIRMATION">
+                  Allow username confirmation
+                </div>
+                <div class="help-text">
+                    If disabled, users logging in will be locked into the username granted by
+                    the registry.
+                </div>
+            </td>
+          </tr>
           <tr ng-show="config.FEATURE_MAILING">
             <td class="non-input">Team Invitations:</td>
             <td colspan="2">