Firefox does not support window.ClipboardItem yet (it is behind
a flag (dom.events.asyncClipboard.clipboardItem) as at version 87.)
so we need to fall back to the normal non-async clipboard copy, that
works in every browser except Safari.
This commit also tests the clipboardCopyAsync function by stubbing out
the clipboard on the window.navigator.
This fixes an issue in the discourse-chat plugin, where the
"Quote in Topic" button errored in Firefox.
Martin Brennan
cecdef83a8