Avoid creating comboboxes

(closes #5568)

Several strategies in here:
- Move uiCombobox() from inside the render function to class variable
- Don't render stuff like the raw tag editor when it's collapsed
- Don't show as many fields/combos on hover
- Don't instantiate fields (like universal/more) until they're actually shown
- Bind the combo on enter selection not on update selection

modules/ui/changeset_editor.js

@@ -11,6 +11,7 @@ import { utilRebind, utilTriggerEvent } from '../util';
 export function uiChangesetEditor(context) {
     var dispatch = d3_dispatch('change');
     var formFields = uiFormFields(context);
+    var commentCombo = uiCombobox(context, 'comment').caseSensitive(true);
     var _fieldsArr;
     var _tags;
     var _changesetID;
@@ -78,8 +79,7 @@ export function uiChangesetEditor(context) {
                     });
 
                     commentField
-                        .call(uiCombobox(context)
-                            .caseSensitive(true)
+                        .call(commentCombo
                             .data(_uniqBy(comments, 'title'))
                         );
                 });