As part of the popover menu redesign, this redesigns the compose send
options popover using the new "popover-menu" tippy theme and improves
accessibility by using appropriate ARIA attributes.
Fixes#25117.
We use "prop-element" class only to handle save-discard widget
and not anywhere else, so there is no need to have this class
for elements that do not use save-discard widget.
Now we show the number of drafts that are addressed to the current
recipient selected in the compose box, if any, in the Drafts button
within parentheses (whether it is next to the Send button, or in the
Send options popover), and explain that it is the number of drafts for
this conversation in the tooltip.
Fixes: #28696.
We need to use the `navigate-link-on-enter` class for the scheduled
messages link in the popover, since it's a popover menu item with a
`href` attribute. The reason for using this class has been documented
in 246c1d7b4e.