Updates changelog for PRs merged in 1.0a2 or 1.0a3
Updates changelog for PRs merged in 1.0a2 or 1.0a3

I did a quick pass to delete things that shouldn’t be in the changelog, like trivial bugfixes that shouldn’t affect users. I also did some quick categorization of upgrades. Eventually we’ll want to merge this list of PRs into the categories below.
jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab