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>

templates/repo/issue/view_content/add_reaction.tmpl

@@ -7,7 +7,7 @@
 		<div class="header">{{.ctx.locale.Tr "repo.pick_reaction"}}</div>
 		<div class="divider"></div>
 		{{range $value := AllowedReactions}}
-			<div class="item reaction tooltip" data-content="{{$value}}">{{ReactionToEmoji $value}}</div>
+			<a class="item reaction tooltip" data-content="{{$value}}">{{ReactionToEmoji $value}}</a>
 		{{end}}
 	</div>
 </div>