Added basic collaboration functionality
- Currently multiple users can insert characters to a shared document
- Can move cursor with left and right arrows and updates on friends' docs
  (TODO: end, home, up arrow, down arrow, mouse)
- On write by one user, every user receives updated document content
  (data.content), their own username and cursorOffset (data.me.username
  and data.me.cursorOffset), and an array of each friend's username and
  cursorOffset.
- Still need to refactor quite a bit.