Support rich text composing in hyperkitty create new thread

Missing this feature was one crucial reason we chose listmonk for announcement type mailing lists. Adding this feature will make it comparable to listmonk in features.

This is crucial when you want to use a send only domain address and incoming mail is handled by another mail server. So you cannot use regular email clients to post to the list and you can post only from the web interface.

In this example, we have lists.puri.sm for regular mailing lists, but we wanted to sent newsletters with @puri.sm address, which was not possible in mailman currently without a lot of hacks in postfix.