Create a new portal for downscaled CMIP6 data

[corviday]

This PR creates a new web portal so users can view and download CMIP6 datasets that have been downscaled with BCCAQ2. The portal is extremely similar to the Downscaled GCMS portal, where users can view and download CMIP5 BCCAQ2 datasets; they use the same all-Canada map component.

This PR also includes a minor update to the mapping code to take advantage of the updated ncWMS. Previously, with ncWMS 1, we could not use the BELOWMINCOLOR parameter to specify what colour zero precipitation should be rendered as when colours were assigned with a logarithmic scale. Now that @rod-glover has updated the PDP to use ncWMS 2, we are able to use this parameter to generate logscaled precipitation maps with no colour discontinuity.

Demo coming.

resolves #250 resolves #252

[corviday]

Demo is up, seems good to me. We're awaiting any UI tweaks from RCI, then we'll merge.

I'm not going to run black on this branch, since that would make rebasing the python3 branch onto it way more annoying. Running black on all the python files is something we've done on several of the PDP components as part of the Big Update, so it would make sense to run black as part of the upgrade to python 3.

[corviday]

UI tweaks requested by RCI:

• [x] revert logscaled precipitation colours back to transparent in cases of low or no precipitation
• [x] format experiment strings like so: Historical, SSP1-2.6
• [x] leave only the first CANESM5 run in the CMIP6 portal
• [x] create a separate page for the rest of the CanESM5 scenarios

[corviday]

• [x] format date on map caption as "human readable".

(I have asked Charles to specify a format, I think it's human readable now.)

[corviday]

This issue tracks data processing for the new portal.

[corviday]

New CanESM5 portal page.

I think the code is ready to go, except then RCI is going to think a little more about the page title. Datasets are still being processed.