Traditional Backfill Missing

[niebloomj]

As pointed out here, traditional Backfill is not yet supported.

This means backfill is impossible on dendrite servers.

I request that we update the slack bridge, specifically the code here, to be like the others, like the newer WhatsApp code here, and support traditional Backfill so we can bring in our historical messages

[niebloomj]

A more relevant backfill code https://github.com/mautrix/gmessages/blob/main/backfill.go

[niebloomj]

I was able to do a traditional backfill after checking out this PR #49