switch to a choice component for work package relation form in pane

frontend/app/templates/work_packages/tabs/_add_work_package_relation.html

@@ -1,24 +1,29 @@
-<form name="add_relation_form">
-  <button class="button"
-          title="{{ btnTitle }}"
-          ng-bind-html="btnIcon + ' ' + btnTitle"
-          ng-click="handler.addRelation(this)"
-          ng-disabled="add_relation_form.$invalid">
-  </button>
-  <ui-select
-    id="relation_to_id-{{ handler.relationsId }}"
-    name="relation[to_id][{{ handler.relationsId }}]"
-    ng-model="$parent.relationToAddId"
-    title="{{ I18n.t('js.field_value_enter_prompt', { field: I18n.t('js.relation_labels.' + handler.relationsId) }) }}"
-    required
-    theme="select2">
-    <ui-select-match>{{$select.selected.to_s}}</ui-select-match>
-    <ui-select-choices
-      refresh-delay="100"
-      refresh="autocompleteWorkPackages($select.search)"
-      repeat="item.id as item in options">
-        <div ng-bind="item.to_s"></div>
-    </ui-select-choices>
-  </ui-select>
-
+<form name="add_relation_form" class="form">
+  <div class="choice">
+    <div class="choice--select">
+      <ui-select
+        id="relation_to_id-{{ handler.relationsId }}"
+        name="relation[to_id][{{ handler.relationsId }}]"
+        ng-model="$parent.relationToAddId"
+        title="{{ I18n.t('js.field_value_enter_prompt', { field: I18n.t('js.relation_labels.' + handler.relationsId) }) }}"
+        required
+        theme="select2">
+        <ui-select-match>{{$select.selected.to_s}}</ui-select-match>
+        <ui-select-choices
+          refresh-delay="100"
+          refresh="autocompleteWorkPackages($select.search)"
+          repeat="item.id as item in options">
+            <div ng-bind="item.to_s"></div>
+        </ui-select-choices>
+      </ui-select>
+    </div>
+    <div class="choice--button">
+      <button class="button"
+              title="{{ btnTitle }}"
+              ng-bind-html="btnIcon + ' ' + btnTitle"
+              ng-click="handler.addRelation(this)"
+              ng-disabled="add_relation_form.$invalid">
+      </button>
+    </div>
+  </div>
 </form>