Index BurnTokens event

[tomquirk]

Following jbx-protocol/juice-interface#2881, would be nice to index BurnTokens event and show it in the activity feed.

Emitted event: https://github.com/jbx-protocol/juice-contracts-v3/blob/297c99dc8e7a179d5ff279459b1aaea860e131b7/contracts/JBController.sol#L697

[peripheralist]

@tomquirk we could also just query transfers to the 0 address