Issue 945: fix jumpy narrative map

[mapmeld]

Fixes #945

On the current site, when you resize the map by resizing the browser, switching between full <=> grid layout, or switch slides in a narrative, maybeInvalidateMapSize sets a 1500ms timer to call map.invalidateSize() and fill in blank map space. This can clash with the narrative's goal to focus the map "viewport" to contain the data.

Proposed solution in narrative mode, map.invalidateSize() happens immediately when we arrive on a new slide. The Leaflet resize event listener will be called after the resize is completed, and this is when we fit viewport to the data.

In free (not-narrative?) mode, the only change is I halved the resize-browser to resize-map delay to 750ms. If you prefer it can be un-changed.

[jameshadfield]

Thanks @mapmeld -- tested and it fixes the issue with no apparent regressions. The map logic is complicated, so thanks for tracking down a fix here ⭐️