Publish mixin feed during CI build

[carolynvs]

This has the main change, which uses porter mixins feed generate to generate the mixin feed and upload it to our blob storage during a CI build.

It also has a bunch of little fixes to the feed generation and search that I found when testing the generate and install commands manually against real URLs.

• Fix call to make temp directory. Oops that worked fine with the unit test, but borked on a real OS. Go figured. 😊
• Don't mark the atom feed as executable when we download it and parse it. That was leftover from when I reused the same code to download a mixin binary.
• Improve cleanup when downloading the atom file. Again, removed some problems from when I reused the same download code for the mixins, for downloading the atom feed file.
• Fix typo in example, so that if they ran it the URL is usable.
• Install the highest semver when asked for "latest" from an atom feed.