add legend to pane 2 maps

[cmkimber]

The maps on pane 2 use circle markers to show their relative frequency of usage as starting or ending points of trips. While the general meaning is likely interpretable without a legend (more intense red -> more usage), a legend for each map would be helpful.

[cmkimber]

rstudio/leaflet#485 is relevant to getting the legend to work

[cmkimber]

Currently the legend is plotted but when the circle markers are updated, the legend is not refreshed but rather an additional legend is added with each change.

[cmkimber]

Using clearGroups with the group for the associated circle markers, which the legend is linked to, does not clear the legend. Added a call to clearControls to clear and rebuild the legend when the map contents change.