Update Gift Sub Response Modes

[gmt2001]

Intentional 2 second delay before thanking gift subs, extended a further 2 seconds if another gift from the same sender comes in

4 modes for handling gift sub notifications

1. Current system: All gift subs get an individual thank-you
2. Single-gifts only: Individual thank-you messages only sent if no Mass Gift event is detected from same sender. The separate mass-gift thankyou setting can still thank them by number of gifts without names, if enabled
3. Combined Message: All gift subs get a thank-you, but if multiple are sent within the delay, it's a single multi-thank-you message similar to welcomeSystem. This message would be configured separately from the single-gift message (Sender just gifted subs to a, b, c, d, e, f, g, and 5 other people! Thank you!)
4. Disabled: No message