return a `Vary: Accept` header for all dual-format endpoints #365 (#13044)

`/users/:user`, `/@:user`, `/notes/:note` return different responses
depending on the request's `Accept:` header. If we don't consistently
return a `Vary: Accept` header, browsers and caching proxies will get
confused, and return AP representations when HTML was requested, or
vice versa.

Co-authored-by: dakkar <dakkar@thenautilus.net>
Co-authored-by: syuilo <Syuilotan@yahoo.co.jp>