Allow user to specify baseurl

[FraMecca]

Hi,
for using this software behind reverse proxy it would be useful to specify a baseurl so that every link in the template is prefixed with the baseurl:
http://example.com/chan ---> root of the imageboard

[gnstaxo]

To be honest, I was not aware of the existence of reverse proxies, thank you for letting me know

[FraMecca]

It wont be that much complex than putting f'{baseurl}/' in front of each decorator and in front of templates hrefs. I don't have the time to open a PR on this right now, sorry.

[FraMecca]

thank you very much for implementing this