Configurable allowed IP / rate limit header

[bjarneo]

So, currently the only support for this repository is the "do-connecting-ip" header for digital ocean to fetch the user IP. However, if people self host, they most likely do not have this header.

To do:
Rewrite this part of the code to accept a string from a ENV var injected to the docker container to decide what header to look for. https://github.com/HemmeligOrg/Hemmelig.app/blob/main/src/server/decorators/allowed-ip.js#L15
Set the default header to "do-connecting-ip".

Note, update this code as well: https://github.com/HemmeligOrg/Hemmelig.app/blob/main/src/server/decorators/rate-limit.js#L21

[bjarneo]

Switch logic for this. Currently it checks an array if the header exists, and if it does, use that header. See connected PR.