binder-examples/appmode needs workaround to enable appmode to work for now

[fomightez]

Bug description

Appmode won't work because it needs to create hidden files and that isn't working now. See here.

Expected behaviour

Launching from https://github.com/binder-examples/appmode should open a session and when I click on appmode button

Actual behaviour

There's an error right now so there is a '400 Bad Request' page when you try to open in appmode.

Here is the issues post about the error at the appmode repo.

How to fix

The workaround is described here.

To my fork of binder-examples appmode, I added a file named jupyter_notebook_config.py with c.ContentsManager.allow_hidden = True as the content and it fixed it.
My version of jupyter_notebook_config.py: jupyter_notebook_config.py

How to show it is fixed

Go to my fork of the repo here and click on launch binder. It will open in appmode as it should.

It would also be nice to update the badge while at it.

[manics]

This will hopefully(!) be fixed when oschuett/appmode#65 (comment) is released

appmode isn't pinned:
https://github.com/binder-examples/appmode/blob/ef9cb16bf8002bde4a2dc39c8c40ed7ebbeab971/environment.yml#L5
but since the image may be cached the most reliable option will be to set a minimum version constraint