Add support for CyberTipLine/PhotoDNA (Image fingerprinting for CSAM detection and reporting)

[paigeadelethompson]

I originally posted this as a feature request to Dendrite but it should probably also be supported for Synapse: matrix-org/dendrite#3416

At least with support for something like this channel admins can vet what home servers they would want to allow federated joins from.

[paigeadelethompson]

This is actually more relevant to synapse though because #synapse-admins:matrix.org is what inspired this idea for me anyway. It's nice that cloudflare has this, but not everybody on Matrix uses cloudflare and the whole thing doesn't do a lot of good unless people can also report abhorrent material.