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

[gripedthumbtacks]

hey, looks like you can patch it up from this URL:

https://aus1.torproject.org/torbrowser/update_2/hardened/

[gripedthumbtacks]

or this one:

https://aus1.torproject.org/torbrowser/update_2/hardened/downloads.json

[adrelanos]

Tor Browser Hardened is dead.

https://lists.torproject.org/pipermail/tbb-dev/2017-February/000454.html

Long live Sandboxed Tor Browser.

https://trac.torproject.org/projects/tor/wiki/doc/TorBrowser/Sandbox/Linux