Change fake and refactor app_bits repo.

- App bits repo is much more tightly scoped
- The App Bits repo has a counterfeiter fake, and lives in its own
  package
- Some callbacks met their demise
- We now have a push actor
- Former responsibilities of the App Bits repo have been divided between
  the App Bits repo, the push command, and the push actor.
- All this should make the future implementation of an "upload bits"
  command much easier/possible.

[#74259334]