Event log refactor

[anp]

Extracting this thought from a discord conversation:

3:36 PM] anp: i'm thinking about maybe trying to extract all of the "github mutation" stuff into an event log
[3:36 PM] anp: and that will give us a place to do better testing bc we could write the github stuff as a consumer of the event log, but there could be others
[3:36 PM] anp: like twitter, rss, and a stub consumer for testing

This could help with @Centril's goal of better testing, and would also make it much easier to improve rfcbot transparency and the correctness of our interactions with github's global mutable state (re https://github.com/anp/rfcbot-rs/issues/228 https://github.com/anp/rfcbot-rs/issues/224 https://github.com/anp/rfcbot-rs/issues/105 https://github.com/anp/rfcbot-rs/issues/207 https://github.com/anp/rfcbot-rs/issues/85).