Deal with the deprecation of the `hardened` channel.

Per: https://trac.torproject.org/projects/tor/ticket/20814#comment:11

The hardened channel for Tor Browser is going away. The sandbox will need updates for this.

Easy changes:

• Make it impossible to install the hardened channel.
• The selfrando specific "disable AESNI in NSS" workaround should apply to the alpha channel.

Release after the hardened removal:

• Remove ASAN specific workarounds.
• Purge the remnants of hardened except for the migration path (Can/should be done immediately if the migration path for sandboxed users is a reinstall).

How the actual migration from hardened to alpha is handled on the sandboxing end is currently an open question. The easy thing to do would be to force a reinstall, but people probably don't realize that a reinstall will obliterate all existing preferences and bookmarks. The more involved fix would be to modify the updater to handle channel migration.