repo: remove unused files

Prevent inlining of HTML

- Links to files that should not be inline are marked download [1]

- Content-Disposition is set to attachment if content type is not
  on whitelist.

[1] https://www.w3schools.com/tags/att_a_download.asp

handlers: rework auth checking

This removes some duplicate code

Finish up documentation for first release

Migrate to Go Module