Consolidate iterations feature flag usage

Also small UI fixup for the collapsed right sidebar

ee/app/assets/javascripts/sidebar/components/iteration_select.vue

@@ -156,12 +156,12 @@ export default {
 </script>
 
 <template>
-  <div class="mt-3">
+  <div>
     <div v-gl-tooltip class="sidebar-collapsed-icon">
       <gl-icon :size="16" :aria-label="$options.iterationText" name="iteration" />
       <span class="collapse-truncated-title">{{ iteration }}</span>
     </div>
-    <div class="title hide-collapsed">
+    <div class="title hide-collapsed mt-3">
       {{ $options.iterationText }}
       <gl-button
         v-if="canEdit"

ee/app/views/shared/issuable/_iteration_select.html.haml

-- if @project.group.beta_feature_available?(:iterations) && issuable_type == "issue"
+- if Feature.enabled?(:group_iterations, @project.group) && @project.group.feature_available?(:iterations) && issuable_type == "issue"
   .js-iteration-select{ data: { can_edit: can_edit, group_path: group_path, project_path: project_path, issue_iid: issue_iid } }