the `ProxyPreserveHost` is necessary otherwise signature checks fail

the `X-Forwarded-Proto` fixes the mastodon emulation (without it, it
tries to talk to `http://sharkey.example` and can fail)

the `ProxyPassReverse` are now the same as in my own configuration,
still not sure how useful they are (they tell apache to rewrite
`location` headers in responses)

thanks to tauroid on Discord for noticing the problems and helping me
debug the configuration