imprv: Allow drag'n'dropping http URIs
This commit makes it possible to drag'n'drop images (and other content types) from firefox (and probably other apps) to send them in a chat.
This does not trigger the file upload dialog but instead sends the URL in a <oob>
tag (if it is not a link to a web page). This is a rather small change but I believe this is very nice UX/QoL change.