bots with mailto: link in html response not working

[adbenitez]

some bots like www gateway bots (bots to fetch preview/snapshots of websites and send them to the users as html messages) use mailto links in html messages so commands inside the html can be forwarded back to the bot chat, when such a mailto is clicked in android, the key contact for the given address is used as the chat where to put the draft to, that already works well, desktop is instead rejecting to use "query contact id by addr" api and showing dialog:

[nicodh]

That was introduced in #5620 assuming that it doesn't make sense to ask for starting a chat based on an email addr without having the key on a chatmail server.

So iiuc we should check first, if there is an existing contact with that address and if so open the "Do you want to chat with" dialog and only if not show this dialog?