Add landing page

[Hashfastr]

Added a basic opt-in landing page for the relay that uses an HTML template embedded into the executable. The landing page is similar to those found with Pleroma relays, it shows the description and a list of subscribed instances. This landing page also implements the relay icon and image into the page along with custom CSS and bootstrap to make things look nice. Also added clarification details that show how to subscribe if you're on Pleroma or Mastodon. It also now links to this repository as well.

Also added a basic Makefile to allow for easy building/installation/upgrade for users.

• [ ] Should the CSS be changed?
• [ ] Are there any details to be added to the landing page?
• [ ] Should the formatting be changed?

See my current running example here: https://relay.an.exchange

[jippi]

@yukimochi any chance for this to land? would be fantastic