Rename this repository to "serverlist"

[luk3yx]

I think the name "serverlist" is much more intuitive than "master-server".

• Why is this a master server when ContentDB isn't? What happens if/when Minetest starts interacting with even more web services?
• Calling this "serverlist" brings the repository name in line with the serverlist_url configuration entry and the first line of README.md.
• GitHub seems to automatically redirect old repository URLs, this shouldn't break anything.

[rubenwardy]

Done, on agreement from sfan5