Miscelaneous server fixes.

- Added try-catch statement surrounding server.listen();
- Added a function for setting up a new user;
- Valid links are now emitted to all hosts rather than only the user that generated the
  link;
- Added sanity check when a socket disconnects (must have been added before
  removing)
Formatting (indentation) fixes.
Break early if node is found when placing on the board.
Improvements in server implementation.

- Added a graph representing the current state of the board;
- Fixed link creation checks;
- Added new functions to handle the creation of entities/links.