Improve course chat on mobile devices/small screens, fix iOS input zoom

We got the feedback of teachers not using the course chat, because it's not behaving nicely on mobile devices and so they are using external tools for synchronous communication. What I see is that the dialog box has a fixed pixel width and height and I hope the user experience on small screens can be improved by tweaking some of the relevant CSS for a better responsiveness.

On a smart phone the chat dialog should fill the entire screen. 

In particular, iOS has a nasty behaviour that zooms OpenOLAT whenever the user inputs something in a form. When the form gets submitted the zoom effect stays which breaks the entire layout. This must be addressed as well