Notification type - join a group requests

[Eldenroot]

Add an option to enable/disable notification for:

• Someone requests to join a group you lead

[Eldenroot]

@lairdshaw this and at least 4 others could be added by integrating another extension from shade. Better to have all inbuild. Anyway one thing needs to be solved - add a new setting type - enable this type of alert in acp but predefined default value for users (enabled/disabled).

[lairdshaw]

@Eldenroot, without intending any offence, I'd rather spend my (limited) time and resources on implementing functionality that isn't already implemented (even if accessing that already-implemented functionality requires installing another plugin).

Anybody else (including yourself) is, of course, welcome to submit a PR integrating this functionality from Shade's plugin (or independently or whatever).