Testing: Basemap fixes and provider credentials

Branch: fix/basemap-providers. This covers #76, #80 and #81.

Most of our basemaps were broken. All six CARTO ones were watermarked without an API key, including Voyager, which was our default, so most sites have been showing a watermarked map. The six Stamen ones fail on any real domain, and Esri DeLorme was retired.

What changed:

• The default basemap is now OpenStreetMap Standard instead of CARTO Voyager. Existing sites on Voyager will see their maps change unless they add a CARTO key.
• There is a new CARTO API key setting.
• If a CARTO or Mapbox basemap is missing its key, the map falls back to the default instead of breaking.
• Stamen is the exception. It needs this site's domain registered with a Stadia account, there is no key to enter for it, and without the registration the map stays blank.
• Base Map settings are now their own section on the config page, and each basemap option names its provider.
• For #80, the Base Map description now recommends two basemaps with high contrast between map features. I explained the reasoning in a comment on that issue.

What to test:

• Each basemap, everywhere a map appears: the browse map, item pages, exhibit pages, the Map tab when editing an item, and the admin side.
• That the config settings still save. I moved fields around on that page, so it is worth checking a few unrelated to basemaps too.
• CARTO with a real API key, Mapbox with a real access token, and Stamen with this site's domain registered with Stadia. Those three need accounts.