Bump to use Jupyter Lab 1.0.1 APIs

[andrewhead]

This PR is a work in progress, in response to https://github.com/microsoft/gather/issues/22

As of the code in the current version of the PR, the dependencies have been updated to Jupyter Lab packages version 1.0.1, and the code has been edited to make sure the tests run. Gather functionality (gather to cells, notebook, and revisions) all appear to work.

There are two remaining issues to be solved before merging:

• [x] How do we publish this so that the new releases can continue to work in versions <= 0.35.4? Will it work in older versions of Jupyter Lab by default? (Check off when we have tested that the default installation of a dev version of the plugin works for both 0.35.4 and a 1.0.x version of jupyterlab)
• [x] Small style fix-ups: something must have changed about the name / location of the notebook icon, as the icon isn't showing up in the UI (see below). At this point, we should just produce our own asset to make sure that we don't depend on the Jupyter Lab version to be able to find the notebook icon (Edit: fixed as of https://github.com/microsoft/gather/pull/26/commits/3c7989d0423226471f09d3473412e11fe9e7b3c3)

[andrewhead]

From the Jupyter Lab gitter, jupyter labextension install will make an effort to install a version of the extension that is compatible with Lab. Sources:

• https://gitter.im/jupyterlab/jupyterlab?at=5d3563728fe53b671db67c35
• https://gitter.im/jupyterlab/jupyterlab?at=5d35d2d1c89880134742f86d