SSB in metamask

An anchor issue for putting [ssb](https://github.com/ssbc/ssb-server/tree/minimal) in metamask
- [x] set up test pub
- [ ] test user invites
- [x] get poc of ssb messages in metamask
- [ ] scale down the ammount of messages persisted to disk
- [ ] inject ssb into the browser like the ethereum provider

stretch goals:
- [ ] ssb keys generated from seed phrases
- [ ] mobile <> extension sync
- [ ] webRtc pubs

Start with the linked ssb-server minimal project and the MetaMask browser-provider integration described in the checklist. Done would require resolving the unchecked items, including user invites, reducing persisted messages, injecting SSB into the browser, and assessing the listed stretch goals.

Written by the indexing model from the issue text.