MSC4076: Add disable_badge_count to pusher configuration

[manuroe]

This PR implements MSC4076: Let E2EE clients calculate app badge counts themselves (disable_badge_count).

The MSC is not merged yet but I have not put this new code under a experimental feature flag. The behavior change is very limited to the homerserver / client / push gateway interfaces. It can only happen if the client asks for it.

Note: this PR now requires setting the experimental feature flag msc4076_enabled - @anoadragon453

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct
  (run the linters)

[manuroe]

The MSC just got the very last change we needed. This PR can be reviewed.

[anoadragon453]

Thanks for your patience on this one. This now LGTM!

[manuroe]

Thanks @anoadragon453 for the help!