Rewriting downloads page for focusing on simpler download experience

Fixes #969 

Ready for review, if not quite ready to merge (I broke the homepage...).

@sethladd @Sfshaza what do you think? I couldn't figure out how/if I wanted to remove the 64-bit links in Windows and Linux pages.

Staged at https://issue-969-new-downloads-page-dot-dart-lang.appspot.com/tools/download.html