This fixes the issue where opening a site, then clicking "Near Me", then
closing the info modal was panning back to the prior site, whereas it
should have stayed at the "Near Me" site. This is "issue C" as discussed
in the comments of GitHub issue #527.

The name change reflects the fact that currently as a user may have
panned around the map by clicking and dragging the map, the `mapCenter`
state was not updated. Instead, the state is updated when e.g. a pin
is clicked, or the "Near Me" button is clicked.

Removed unnecessary if-else condition from the reducer, and
transitioned to the `createAction` syntax.