Implement kicking of users.

Kicking in Eleven blocks a user from a room permanently, they can't re-join right afterwards. However, we still need to persist the kicked users list to disk.
Allow specifying the accounts file as the first command line argument.
We now persist each channel's list of kicked users.

The argument to the server is now a settings *folder* to which it writes both the accounts file and a file with the list of kicked users for each channel.