Add option to choose Enter key to produce line feed or send message

[Pohli]

I can’t enter multiline messages (either Enter key or copy/paste line feed sends first line anyway and inserts line feed before following message).
Please add an option in Settings or near the Send button to change behaviour of the Enter key.

[b100dian]

This is a good idea, since destkop Shift + Enter is not accesible on mobile phones. This means however that there's a such setting support on hydrogen-web (see hydrogen.element.io deployment). We might look into this to contribute upstream