Make issue meta dropdown support Enter, confirm before reloading (#23014)

As the title. Label/assignee share the same code.

* Close #22607
* Close #20727

Also:

* partially fix for #21742, now the comment reaction and menu work with
keyboard.
* partially fix for #17705, in most cases the comment won't be lost.
* partially fix for #21539
* partially fix for #20347
* partially fix for #7329

### The `Enter` support

Before, if user presses Enter, the dropdown just disappears and nothing
happens or the window reloads.

After, Enter can be used to select/deselect labels, and press Esc to
hide the dropdown to update the labels (still no way to cancel ....
maybe you can do a Cmd+R or F5 to refresh the window to discard the
changes .....)


This is only a quick patch, the UX is still not perfect, but it's much
better than before.


### The `confirm` before reloading

And more fixes for the `reload` problem, the new behaviors:

* If nothing changes (just show/hide the dropdown), then the page won't
be reloaded.
* If there are draft comments, show a confirm dialog before reloading,
to avoid losing comments.

That's the best effect can be done at the moment, unless completely
refactor these dropdown related code.

Screenshot of the confirm dialog:

<details>


![image](https://user-images.githubusercontent.com/2114189/220538288-e2da8459-6a4e-43cb-8596-74057f8a03a2.png)

</details>

---------

Co-authored-by: Brecht Van Lommel <brecht@blender.org>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

web_src/js/features/aria.js

@@ -81,7 +81,8 @@ function attachOneDropdownAria($dropdown) {
   $dropdown.on('keydown', (e) => {
     // here it must use keydown event before dropdown's keyup handler, otherwise there is no Enter event in our keyup handler
     if (e.key === 'Enter') {
-      const $item = $dropdown.dropdown('get item', $dropdown.dropdown('get value'));
+      let $item = $dropdown.dropdown('get item', $dropdown.dropdown('get value'));
+      if (!$item) $item = $menu.find('> .item.selected'); // when dropdown filters items by input, there is no "value", so query the "selected" item
       // if the selected item is clickable, then trigger the click event. in the future there could be a special CSS class for it.
       if ($item && $item.is('a')) $item[0].click();
     }