Add "default" account data feature

[Half-Shot]

As part of the work required for element-hq/element-meta#2787. The intention is for server admins to set default configuration for viewing media content that may be overridden by users.

This adds the ability to set account data for all users on the server via a config, that will be used if no account data is already set for a given key.

Things to do:

• Configuration
• Support basic initial syncs
• Support sending deltas of config changes to users
• Handle "deleted" account data.

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)