This commit ports the feature by @chapoi that was
previously a theme component in core.
A new post_menu button, copyLink, is added and used
as the default instead of share.
copyLink, on desktop, will copy the link of the post
to the user's clipboard and show a nice 'lil animation.
On mobile the native share menu will be shown.
If site owners want the old behaviour back, they just
need to change the post_menu site setting to use
the share button instead of copyLink.
