Add option to show list on index page

Fixes: https://todo.sr.ht/~sfr/cassini/4
Remove ioutil dependency

as of Go 1.16, ioutil.ReadFile links to os.ReadFile, which I already
had imported.
Document functions according to godoc spec
Store list last modified time

allows updating the list without restarting the server.

Fixes: https://todo.sr.ht/~sfr/cassini/2
Correctly figure out if capsule is last in ring
Handle root path without trailing slash
Add systemd service file
Add OpenRC init script
Link to sr.ht project page
Serve index page on root
Log and exit after serving random page
Remove duplicate default text
Add contributing information
Create 2D array of username and URL
Change project name to cassini
Load TLS and listen over TCP