Add role assignment logic

[cascer1]

Bot should be able to assign a configured role, based on whether a user has another configured role or not.

For example:

• Any user that sends a message AND has the approved role should be given the Halloween 2018 role

sub-tasks:

• Store a list of roles that may grant a reward
• For each of these roles, store a list of potential rewards
• For each reward, store a list of roles a user must have to receive it

[cascer1]

Rewards will be stored in the format <required>:<reward>