Thunderbird: introduce THUNDERBIRD_USER_DIR variable and ~/.mozilla-thunderbird/ support
Old enough Thunderbird installations still store their profiles in ~/.mozilla-thunderbird/ directory, with symlinks in place, but AppArmor does not follow symlinks.
In addition to supporting ~/.mozilla-thunderbird/, introduce THUNDERBIRD_USER_DIR variable to reduce code duplication.
Bug-Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1030532