Refactored the server into many smaller functions to make it easier to
understand all the different code paths.

- added a Content-Disposition header, which tells the browser the
file's name, since we use urls like http://localhost:port/0 <-- no
human-readable file name
- Server returns valid HTML documents (with all the required tags) now.
- Return 204 status for OPTIONS request
- reduce access-control-max-age to chromium max of 600s
- respond to OPTIONS requests that lack
'access-control-request-headers' (before they were treated as GET)
- return '405 invalid verb' for all other verbs

For: brave/browser-laptop#6737