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

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

@@ -1,10 +1,6 @@
-<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.$invalid">
-  </button>
+<form name="add_relation_form" class="form">
+  <div class="choice">
+    <div class="choice--select">
       <ui-select
         id="relation_to_id-{{ handler.relationsId }}"
         name="relation[to_id][{{ handler.relationsId }}]"
@@ -20,5 +16,14 @@
             <div ng-bind="item.to_s"></div>
         </ui-select-choices>
       </ui-select>
-
+    </div>
+    <div class="choice--button">
+      <button class="button"
+              title="{{ btnTitle }}"
+              ng-bind-html="btnIcon + ' ' + btnTitle"
+              ng-click="handler.addRelation(this)"
+              ng-disabled="add_relation_form.$invalid">
+      </button>
+    </div>
+  </div>
 </form>