Replace HOST_URL by Host header

[Fijxu]

Currently, each invidious process is fixed to it's domain config value which makes it really difficult for anyone to host Invidious using multiple domains, Tor and I2P.

If someone wants to host Invidious on Tor, a dedicated Invidious instance for it needs to be made setting domain to the Tor address, and the same goes for I2P.

On some places, HOST_URL is still used and it shouldn't be changed, like for example:

invidious/src/invidious/helpers/utils.cr

Line 297 in adcdb8c

"hub.callback" => "#{HOST_URL}/feed/webhook/v1:#{time}:#{nonce}:#{OpenSSL::HMAC.hexdigest(:sha1, key, signature)}",

If this get merged when it's finished, it shouldn't break the setup of anyone using NGINX since the invidious documentation already mentions the use of the Host header on the NGINX configuration: https://github.com/iv-org/documentation/blob/65ecfccb10f15b4b9ce95c03b0d6d6ebd88bca7a/docs/nginx.md?plain=1#L24

Consider this a WIP since it's a big change that could break a lot of things, however, most of the changes made on the first commit are already applied to my Invidious fork at https://git.nadeko.net/Fijxu/invidious for a very very long time already, multiple domains works fine and Tor works fine.