Sight + attack

- Implement the attack when the fighter is part of a guild
- load the default avatar 1.png if the player’s avatar does not exist.
It do so on the sight page, on the board, and on the fighter page.
- Modify a bit the field attack message and move message, so they take
place event if they are empty.
- Add message displaying above the board on the sight view every time
you’re being attacked.
- Modify the message displayed in the messages table when its empty.
- Modify the board style in custom.css, so the fighter move keep the
board still.