Hide "Start Build" button in builds tab if not accessible

While the build cannot be started, and, in fact, the only way to see the button is to *manually* set the `tab=` query param, better to hide it

Fixes https://jira.coreos.com/browse/QS-67

static/directives/repo-view/repo-panel-builds.html

@@ -1,7 +1,8 @@
 <div class="repo-panel-builds-element">
   <div class="feedback-bar" feedback="feedback"></div>
   <div class="tab-header-controls">
-    <button class="btn btn-primary" ng-click="showNewBuildDialog()" ng-if="!repository.tag_operations_disabled">
+    <button class="btn btn-primary" ng-click="showNewBuildDialog()" ng-if="!repository.tag_operations_disabled"
+            quay-show="repository.can_write && Features.BUILD_SUPPORT">
       <i class="fa fa-play"></i> Start New Build
     </button>
   </div>