Revert staging upgrade to z2jh master

Reverts berkeley-dsep-infra/datahub#3417 and
berkeley-dsep-infra/datahub#3420. They both
work, and I think we're good to go - but I didn't want to deploy
it just now as I'll be asleep soon.

Ref berkeley-dsep-infra/datahub#3418

infra-requirements.txt

@@ -8,16 +8,17 @@
 # everwhere one by one.
 
 # FIXME: Freeze this to get exact versions of all dependencies
-notebook==6.4.11
-jupyterlab==3.4.2
+notebook==6.4.7
+jupyterlab==3.3.0
 jupyterlab-link-share==0.2.4
-retrolab==0.3.21
+nbconvert==6.4.0
+retrolab==0.3.18
 nbgitpuller==1.0.2
 jupyter-resource-usage==0.6.1
 # Matches version in images/hub/Dockerfile
-jupyterhub==2.3.0
+jupyterhub==1.5.0
 appmode==0.8.0
-ipywidgets==7.7.0
+ipywidgets==7.6.5
 otter-grader==3.1.4
 jupyter-tree-download==1.0.1
 git-credential-helpers==0.2
@@ -30,4 +31,4 @@ popularity-contest==0.4.1
 # RISE is useful for presentations - see https://github.com/berkeley-dsep-infra/datahub/issues/2527
 RISE==5.7.1
 # syncthing for dropbox-like functionality
-jupyter-syncthing-proxy==1.0.3
+jupyter-syncthing-proxy==1.0.1