Allow choice between "release", "alpha", and "hardened" channels

[micahflee]

Now that Tor Browser Launcher checks uses the release channel XML file to learn the latest version to download instead of the RecommendedTBBVersions file, we should have a much more stable way of learning what version to install.

Here are the XML file URLs for release and alpha:

https://dist.torproject.org/torbrowser/update_2/release/Linux_x86_64-gcc3/x/en-US https://dist.torproject.org/torbrowser/update_2/alpha/Linux_x86_64-gcc3/x/en-US

And here's the only one I found for hardened, but it includes the version 5.5a4 in the URL, so I doubt it's going to remain this URL forever:

https://dist.torproject.org/torbrowser/update_2/hardened/5.5a4-hardened-linux64-ALL.xml