!hubs subscribe for opt-in channel notifications

[robertlong]

I'd like to propose adding a new command to opt-into channel notifications when an event is starting.

New Commands:

• !hubs create-subscriber-role
  Creates the subscriber role for the bound channel if it doesn't already exist. For example if the channel is named #meetup the @meetup-subscribers role will be created. Only server admins have access to this command. If the bot doesn't have permissions to create/modify roles, the bot will return an error message.

• !hubs subscribe
  Adds the current user to the subscriber role for the bound channel. If there is no subscriber role, the bot will return an error message.

• !hubs unsubscribe
  Removes the current user from the subscriber role created for the bound channel. If there is no subscriber role, the bot will return an error message.

Modified Commands:

• !hubs notify set <datetime>
  If the bound channel has a subscriber role then the bot notifies that role instead of @here

Server admins can also use the @<channel>-subscribers role to send notifications whenever they want.

Server admins will need to enable the MANAGE_ROLES permission for the bot to use this feature.