[REVIEW ONLY] - All downloads menu from downloads.brackets.io

In light of adobe/brackets#7239 and various discussion I've seen about a new downloads page (which I presume to be this), I recreated the All Downloads menu from http://downloads.brackets.io, using a mix of existing and new code. I also performed some minor `"use strict";` and other fixes. 

* Uses GitHub Release links for downloads. Yes, it would require having to go back and upload all the binaries for them, but GitHub Releases was built for this sort of thing.
* Copy relevant CSS from  http://downloads.brackets.io and reused it here. I also added a small CSS fade transition to match the shinyness of the site.
* Uses the preexisting `.json` file for data

**Issues**

- [ ] `window.version_url` is not ideal, IMO. I would think there is an easier way to make that a global variable.
- [ ] Most of the code would be better abstracted into a .js file for use on here and the index.
- [ ] The HTML should be renamed to `index.html`
- [ ] The dropdown menu is _really_ long, and will only get longer with more releases.
- [ ] The information in the right column is out of date.
- [ ] I had a hard time with the menu placement. I ended up leaving it in the same location as downloads.brackets.io, but my initial plan was to center it under the download button. It worked except when I tested it on smaller screens. Some media queries would have taken care of that, but I didn't feel up to it.