Download Archive page

This PR is not perfect yet, but you can look at the code, and the staged "Downloads archive" page:

https://downloads-archive-dot-dart-lang.appspot.com/tools/download_archive/

@sethladd et. al. please throw me your feedback!

Personally, I like the filter functionality and general look, but the table cells on the left look very spaced out because of all of the vertical content on the right... Not sure how I feel about that.

This is made as a dart web app, so there are a ton of files. I'm happy to simplify to a single .dart file with no web/ directory, etc. I dunno if there is a general process for the little dart apps at dartlang.org.