create special editor for opening files that end in ~

[williamstein]

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.

[williamstein]

What jupyterLab does: it seems to just hide ~ files. ugh. that's not good.