Host the explorer html ourselves

[robvanmieghem]

Drop the need for caddy,

• investigate if we can import the caddy code for let's encrypt license wise

[robvanmieghem]

caddy is apache2, so it's fine

[robvanmieghem]

according to the caddy comment in the tls package, this should be fairly easy:
https://github.com/mholt/caddy/blob/master/caddytls/tls.go

[DylanVerstraete]

PR is ready, needs to be tested in production

#511