Add site with redirects for consistent latest-release URLs

[verythorough]

This adds a site folder with a single _redirects file. When published to Netlify, this generates redirects from consistent "latest" URLs to the direct download links for the latest release.

You can test the redirects in the branch deploy:

https://latest-release-redirects--cli.netlify.com/download/latest/mac
https://latest-release-redirects--cli.netlify.com/download/latest/linux
https://latest-release-redirects--cli.netlify.com/download/latest/windows
https://latest-release-redirects--cli.netlify.com/download/latest/source-zip
https://latest-release-redirects--cli.netlify.com/download/latest/source-tar

All of these redirects are 302 temporary, since they'll change with each new release. At the moment, the change will have to be manual, but we can add a script to automate it later.

I also added a couple of catch-all redirects pointing to the releases page and the repo readme, in case they're useful. You can test these, too:

https://latest-release-redirects--cli.netlify.com
https://latest-release-redirects--cli.netlify.com/download

[rybit]

lgtm ~ do you want to add a note that they're only 64 bit versions? We don't build the other versions for the time being

[verythorough]

@rybit Yeah, I was thinking about whether to make the path end mac64 or something, but decided that it's unlikely that we'll release 32-bit versions, and if we do, they'll be the less commonly used version, and they'd be ok being called mac32 while the 64-bit version is still just mac.

As for adding a note, yeah, it makes sense to add that to the docs update I'm working on now.

[brycekahle]

Apple stopped allowing 32 bit apps in the MacOS store in January 2018, so I don't think we need to build a 32 bit version.

[rybit]

sounds good. I am merging this so you're unblocked. We can discuss automation as it comes up later.