text: Title: Error setting template paths; Voila fails to render
  Body: I'm running Voila to serve a dashboarding notebook to users via Jupyter Hub, using a proxy configuration as follows:
  
  `c.ServerProxy.servers = { 'voila':
          { 'command': ['voila', '--debug', '--enable_nbextensions=True',  '--MappingKernelManager.cull_interval=60', '--MappingKernelManager.cull_idle_timeout=120', '--MappingKernelManager.cull_busy=True', '--MappingKernelManager.cull_connected=True', '--no-browser', '--port', '{port}', '--base_url', '{base_url}voila/', '--server_url', '/', '/srv/voila/goldmine.ipynb']
            }
  }`
  
  This has worked perfectly for many months.
  
  After a recent pip upgrade, only 1 of my users is now able to successfully render the notebook page using Voila; everybody else receives a `jinja2.exceptions.TemplateNotFound: voila.tpl` error. 
  
  In comparing the --debug logs for successful and failed scenarios, it appears that the template paths are firstly set correctly (in all cases):
  
  Sep 01 03:46:03 ip-10-0-0-126 bash[19856]: [Voila] **nbconvert template paths:
  Sep 01 03:46:03 ip-10-0-0-126 bash[19856]:         /opt/tljh/user/share/jupyter/voila/templates/default/nbconvert_templates**
  Sep 01 03:46:03 ip-10-0-0-126 bash[19856]: [Voila] **template paths:
  Sep 01 03:46:03 ip-10-0-0-126 bash[19856]:         /opt/tljh/user/share/jupyter/voila/templates/default/templates**
  Sep 01 03:46:03 ip-10-0-0-126 bash[19856]: [Voila] static paths:
  Sep 01 03:46:03 ip-10-0-0-126 bash[19856]:         /opt/tljh/user/share/jupyter/voila/templates/default/static
  Sep 01 03:46:03 ip-10-0-0-126 bash[19856]:         /opt/tljh/user/lib/python3.6/site-packages/voila/static
  
  But for sessions/users that fail, template paths is subsequently prepended with the user's ~/.local path before pre=processing:
  
  **Sep 01 03:46:04 ip-10-0-0-126 bash[19856]: [Voila] Template paths:
  Sep 01 03:46:04 ip-10-0-0-126 bash[19856]:         /home/jupyter-dschofield/.local/share/jupyter/nbconvert/templates/html**
  Sep 01 03:46:04 ip-10-0-0-126 bash[19856]:         /opt/tljh/user/share/jupyter/nbconvert/templates/html
  Sep 01 03:46:04 ip-10-0-0-126 bash[19856]:         /usr/local/share/jupyter/nbconvert/templates/html
  Sep 01 03:46:04 ip-10-0-0-126 bash[19856]:         /usr/share/jupyter/nbconvert/templates/html
  Sep 01 03:46:04 ip-10-0-0-126 bash[19856]:         /opt/tljh/user/share/jupyter/nbconvert/templates/lab
  Sep 01 03:46:04 ip-10-0-0-126 bash[19856]:         /opt/tljh/user/share/jupyter/nbconvert/templates/base
  Sep 01 03:46:04 ip-10-0-0-126 bash[19856]:         /home/jupyter-dschofield/.local/share/jupyter
  Sep 01 03:46:04 ip-10-0-0-126 bash[19856]:         /home/jupyter-dschofield/.local/share/jupyter/nbconvert/templates
  Sep 01 03:46:04 ip-10-0-0-126 bash[19856]:         /home/jupyter-dschofield/.local/share/jupyter/nbconvert/templates/compatibility
  Sep 01 03:46:04 ip-10-0-0-126 bash[19856]:         /opt/tljh/user/share/jupyter
  Sep 01 03:46:04 ip-10-0-0-126 bash[19856]:         /opt/tljh/user/share/jupyter/nbconvert/templates
  Sep 01 03:46:04 ip-10-0-0-126 bash[19856]:         /opt/tljh/user/share/jupyter/nbconvert/templates/compatibility
  Sep 01 03:46:04 ip-10-0-0-126 bash[19856]:         /usr/local/share/jupyter
  Sep 01 03:46:04 ip-10-0-0-126 bash[19856]:         /usr/local/share/jupyter/nbconvert/templates
  Sep 01 03:46:04 ip-10-0-0-126 bash[19856]:         /usr/local/share/jupyter/nbconvert/templates/compatibility
  Sep 01 03:46:04 ip-10-0-0-126 bash[19856]:         /usr/share/jupyter
  Sep 01 03:46:04 ip-10-0-0-126 bash[19856]:         /usr/share/jupyter/nbconvert/templates
  Sep 01 03:46:04 ip-10-0-0-126 bash[19856]:         /usr/share/jupyter/nbconvert/templates/compatibility
  
  Of course, the default template does not exist under the first path (/home/jupyter-dschofield/.local/share/jupyter/nbconvert/templates/html) and therefore rendering fails with the Jinja template not found issue.
  
  This does not appear to be a permissions issue and nothing else that I'm aware of has changed system-wide.
  
  
  async-generator==1.10
  ipykernel==5.3.0
  ipympl==0.5.6
  ipysheet==0.4.4
  ipython==7.14.0
  ipython-genutils==0.2.0
  ipytree==0.1.8
  ipywidgets==7.5.1
  Jinja2==2.11.2
  jupyter-client==6.1.3
  jupyter-core==4.6.3
  jupyter-server==0.1.1
  jupyter-server-proxy==1.2.0
  jupyterhub==1.0.0
  jupyterhub-idle-culler==1.0
  jupyterlab==2.2.4
  jupyterlab-pygments==0.1.1
  jupyterlab-server==1.2.0
  nbclient==0.4.3
  nbconvert==5.6.1
  nbformat==5.0.6
  notebook==6.0.3
  Pygments==2.6.1
  tornado==6.0.4
  traitlets==4.3.3
  voila==0.1.22
  widgetsnbextension==3.5.1
  