web/download: remove developer repos and cleanup

* Showing the developer repos is very confusing to newcomers to this website
* Half of the developer repos are dead.
* There are 599 forks on GitHub, many of them have constructive edits, and it
  is impossible to list them all out.
* Also adds a dedicated "mirror" section.
* Fix GitHub snapshot URL.
* The CSS changes are needed to keep the table as pretty as it is now.
  Suggestions welcome.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
web/olddownload: add FFmpeg 0.9.4