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
