extra tags exclusive to Ko-fi subscription events.

### Subscription Tags
- `[[[event.tier_name]]]` :: Subscription tier name
- `[[[event.is_first_sub]]]` :: "1" if first payment, "0" otherwise
- `[[[event.is_subscription]]]` :: Always "1" for subscription events

note: these tags stack on top of [[all-ko-fi-events|All Ko-fi Events]] + [[ko-fi-donation-and-subscription-events|Ko-fi Donation & Subscription Events]] and only appear on Ko-fi subscription-type events.