Fix jquery atwho implementation

- Options in the <code>@mention</code> flyout can now be selected with a mouseclick.
This was already the librarys default, but "overridden" by the
display_timeout setting of 0 in global.js.
(The inputs blur event is
triggered before the click event, closing the flyout before anything
can be selected. This is the reason for the timeout being there.)

- The tab key no longer closes the flyout, but moves to the next item.
This required 2 minimal changes in the library.

Fixes https://github.com/vanilla/vanilla/issues/2201