ghost.svg caching

[Ivan-Chupin]

Remark42 currently returns the "cache-control: max-age=3600, no-cache" HTTP header for the ghost.svg file. Using the "no-cache" directive is pointless and only creates useless traffic. It would be correct to use the "cache-control: max-age=3600" header, as for all other avatars.

[paskal]

It's the same for all frontend files, as ghost.svg is one of the static frontend files. I vote against introducing granular control of headers per frontend file just for the sake of a single 2kb file.