v1.6.1 — "no basemap" actually removes the basemap

A one-fix release on top of v1.6.

"No basemap" actually removes the basemap

Choosing it in the portal editor appeared to do nothing — and the editor was innocent.

The editor's preview is a real published bundle in an iframe, which is what makes it faithful.
So what the preview shows is decided at publish, in the bundle builder. __none__ is deliberately
not in the basemap catalog — there is no service behind it; it is the absence of one — so the
lookup found nothing, the repoint step never ran, and the template's own basemap stayed baked in.

The runtime switcher could still turn the basemap off afterwards, which is exactly why it seemed
that only the published portal honoured the setting. It didn't: the stored choice was being
ignored in both places, and the portal only looked right because you had switched it off by hand.

The basemap is now taken out of the style at publish rather than hidden on load, on a white
ground — with no painted ground, what shows through a map is the canvas, which is black. No flash
of a map nobody asked for, and the switcher opens with None ticked.

Only the template's sources are removed: a data source that happens to be called basemap is the
map itself. Everything else is unchanged — a real basemap still repoints, a portal that chose
nothing looks exactly as it did, and an unknown id (a typo, or a basemap since removed from the
catalog) leaves the template's basemap rather than silently publishing a blank map.

Upgrading

docker compose build geodeploy-api && docker compose up -d --force-recreate geodeploy-api celery,
or Update now in Settings. Existing published portals keep their current basemap until they are
re-published.