create special editor for opening files that end in ~

These get created by rsync when making backups, etc., and cocalc sometimes uses rsync for sending files around right (but won't in cocalc2).  In any case, maybe we should have a special editor when you open, e.g., a.ipynb~, which suggests renaming the file to a.ipynb instead, then opening that.  Right now you just get the "i don't know what to do with this file" message.