Stable nbconvert acceptance test

Change the `nbconvert` test so that it converts a notebook with no
Hoogle (which changes over time) and no images (which differ on
different systems).

This should reduce the number of surprise Travis test failures.

Dockerfile refactor

rm ihaskell-display/ihaskell-*/stack.yaml

Andrew Gibiansky created all these package stack.yamls with this commit:

> commit f9846b6b
> Author: Andrew Gibiansky <andrew.gibiansky@gmail.com>
> Date:   Tue Jun 7 17:26:47 2016 -0700
>
>     Separate stack.yaml for each package for simplicity

The resolvers for all these stack yamls originally agreed with the
root stack.yaml, but now they're way off, apparently out of neglect.
Let's just delete them.

This Dockerfile gives us more control and isolation of dependencies
during development.

Base image `FROM ubuntu` instead of `fcpo/stack-build`.  We will use the
Stack and GHC on the host system, don't need it installed in the image.
The image is just for isolating the system package dependencies. Stack
will isolate the Haskell package dependencies (and the pip and npm
dependencies).

This gives is a single authority for the resolver, which is
the `stack.yaml`. We no longer need to manually make the `stack.yaml`
and `Dockerfile` resolvers agree.

Don't install `jupyter` in the Docker image, we'll use `stack --docker` to
install it later. This allows developers to choose the version of the jupyter
dependencies.

Add the `docker` section to `stack.yaml`, disabled by default.

__Motivating example:__ ipywidgets version 7 has changed its protocol.
We want to upgrade ihaskell-widgets, which works only on ipywidgets
version 6. To install ipywidgets version 6, we need to downgrade Python to 3.5.
We can use `stack --docker` to isolate a development environment in
which ihaskell-widgets is working, and then upgrade it to the latest
versions.

Fix compliance with Jupyter 5.0 spec for idle messages

jrg/with anim fps

Passthru of some release internal expressions

This is so that these can be reused (e.g.: for
specifying a nixos jupyter service kernel).

labextension npm dependency bounds loosen

Relax upper version bounds on the jupyterlab alpha dependencies
for ihaskell_labextension. It looks to me like these version bounds
were intended to allow the entire major alpha 0.*.* version range, but
the the prefix caret only includes minor versions if the major version
is 0.

Let's set the upper bound to 2.0.0, because according to SemVer, that
is the next version number which we would expect to have breaking
changes.

Attempt to set error status

Change extend to override in release.nix