Allow to configure custom submission message heading

[susnux]

Follow up on #1659

We can maybe allow to configure not only the message but also the heading of the submission message.
I see two options:

1. Add an input for the heading
2. Extract the heading from the message

I do not like 1. as it introduces not really necessary fields on the DB.
For 2 we could use the Markdown and check whether the first element is an heading and if yes then use that one as the heading. I think this makes the most sense.

[Chartman123]

What about omitting the default title as soon as a custom submission message is provided? Then it would be completely up to the Form creator how the sumission message will look like.

[susnux]

What about omitting the default title as soon as a custom submission message is provided? Then it would be completely up to the Form creator how the sumission message will look like.

Well thats so simple, why did I not think of that 🙈
Sound quite good :)