Client feature: Mat Create + Join

[JvmName]

Uses #1. A client should be able to:

1. Create a new Mat
   • main page -> "New Mat" -> enter name -> receive code -> tell other judges the code
2. Join an existing Mat
   • main page -> "Join Mat" -> enter code
3. Join a Mat as a Viewer
   • TBD if it's the same code or a "viewer" code