Add authentication

[Serneum]

• [x] Add usernames and passwords to players
• [x] Add a "sign up" page
• [x] Add a "log in" page
• [x] Remove the "Add a player" link on the main page
• [x] All 'accounts' are automatically added to the Inactive list as they are created
• [x] Remove the Delete button from players

[Serneum]

http://nithinbekal.com/posts/phoenix-authentication/

[Serneum]

We will need to delete all existing players as part of the migration

[Serneum]

Should we rename Player to User?