Implemented maximum player game-setting.

It is set using the query-parameter 'm'. There is no GUI for it yet.
Correct error messages on the client...

...when the game is full or got an invalid ID in the websocket
connection (although that should never happen!)
Probably 'alert' is not cool and we should redirect to the mainview and
show a pretty error message there.
Show an error message on the start view if needed

Using bootstrap alerts and showing the html code passed in the 'errmsg'
query parameter. Happens when:

- joining an inexistent game
- joining a full game