Sync GitHub issues with BridgeBot

[michielbdejong]

Working on this in BridgeBot.

• [x] Fetch from GitHub to DXOS
• [ ] Add diff-stream
• [ ] Add Cambria
• [ ] Process events from DXOS
• [ ] Full two-way sync between GitHub and DXOS

[michielbdejong]

This needs unit tests:

• [ ] fetch/push from scratch
• [ ] fetch/push update - new issue
• [ ] fetch/push update - updated title
• [ ] fetch/push update - new comment
• [ ] fetch/push update - updated comment
• [ ] fetch/push update - updated issue status
• [ ] fetch/push update - issue removed

[michielbdejong]

It seems there are events for when e.g. a comment is added but I can't find an event for when a comment is updated - that might require diff-stream

[michielbdejong]

Ah, found it, in webhook docs: comment, edited

This means (and that makes sense) that BridgeBot should expose a https endpoint to receive webhooks

[michielbdejong]

Creating https://github.com/federatedbookkeeping/github-dxos for this