Make dropdowns on new issue page consistent

app/assets/stylesheets/framework/dropdowns.scss

@@ -728,6 +728,8 @@
 @mixin new-style-dropdown($selector: '') {
   #{$selector}.dropdown-menu,
   #{$selector}.dropdown-menu-nav {
+    margin-bottom: 24px;
+
     li {
       padding: 0 1px;
 

app/assets/stylesheets/pages/issues.scss

@@ -143,8 +143,12 @@ ul.related-merge-requests > li {
   }
 }
 
-.issue-form .select2-container {
-  width: 250px !important;
+.issue-form {
+  @include new-style-dropdown;
+
+  .select2-container {
+    width: 250px !important;
+  }
 }
 
 .issues-footer {