Flag fallback directory mirrors in Atlas

Tor keeps a list of fallback directory mirrors in its source code.

consensus-health flags these mirrors using the synthetic flag "FallbackDir".

Could Atlas do something like this, too? (Like the Not Recommended flag?)

The list is updated every 2-6 months: https://gitweb.torproject.org/tor.git/tree/src/or/fallback_dirs.inc

And then parsed and included in stem in a friendlier format: https://gitweb.torproject.org/stem.git/tree/stem/descriptor/fallback_directories.cfg