DropdownMenu component is meant as a way to describe the content of menus.
Syntax:
```
<DropdownMenu as |dm|>
<dm.item class="test">
First
</dm.item>
<dm.divider class="foo" />
<dm.item class="bar">
Second
</dm.item>
</DropdownMenu>
```
Adds the new quick menu for bookmarking. When you bookmark
a post (chat message behaviour will come later) we show this new quick
menu and bookmark the item straight away.
You can then choose a reminder quick option, or choose Custom... to open
the old modal. If you click on an existing bookmark, we show the same quick menu
but with Edit and Delete options.
A later PR will introduce a new bookmark modal, but for now we
are using the old modal for Edit and Custom... options.
