Update the Stamen map provider #507
Description
Per the announcement at http://maps.stamen.com/stadia-partnership/, the Stamen map tiles are now hosted by Stadia Maps.
I took a look around the code and noticed that the Stamen layers are provided via OpenLayers. We submitted patches to OpenLayers, which were released in version 8 on September 1st: https://github.com/openlayers/openlayers/blob/v8.0.0/changelog/upgrade-notes.md.
Alternately, if upgrading is not a possibility, you could switch to using XYZ layers. We have documentation on doing this here: https://docs.stadiamaps.com/tutorials/raster-maps-with-openlayers/#openlayers-versions-older-than-v800.
Finally, I would suggest adding an optional API key parameter in the configuration for Neatline with a link to https://docs.stadiamaps.com/authentication/. Most web deployments do not need an API key; just an an account with a registered domain.