feature request: add support for different roles in a task coded into the token

[sebag90]

All users are equal in slurk and if a task requires users to have different roles, the only options are:

• the bot can randomly assign roles to the users of a task
• users have to enter a command to assign users (es. recolage bot)
• solutions that require modifications to the concierge bot:
  • create tokens for different users with different rights (es. one with and one without send_command right), once two users with different rights have been paired, have the bot modify the rights to fit the task
  • use a landing page that automatically assign a username and a token to a new users and code the role into the username

Proposal:

• add a new attribute num_roles (int) to tasks
• add the parameter role (int) to tokens - the mapping between role number and actual role will then be defined in the task bot
• modify the concierge bot in a way that, if a task has more than one role, it will only pair users once a user for each role connected to the waiting room