Don't allow tagnames longer than 255 bytes

[SnackEngineer]

If a user tries to add a tag with a length of over 255 bytes return BAD_REQUEST, since this is not allowed by the matrix specification.
(see: https://spec.matrix.org/v1.15/client-server-api/#events-14)

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)

[anoadragon453]

Thanks for doing this! A small gotcha and nit below, but otherwise LGTM!

[anoadragon453]

LGTM, thank you!

[anoadragon453]

(CI failures are unrelated to this PR.)