Add custom relay page

[kuba-04]

Hi, first of all thank you for this repo. I am running my own relay and I use your repo for it. The thing I believe could be improved from my stand point is the relay page. There is a plain string in the code and there are suggestions here to hardcode html. I am not a fan of hardcoding such things, therefore I propose adding a possibility to use your own external html file that you can mount to your docker container. In this way you can easily customize your page according to your liking. Please let me know what you think (I am new to rust btw so any suggestions are welcome!)

[scsibug]

Thanks, looks good. Future improvement might cache the HTML instead of reading it each time.

Merged.