Allow CircleMarker to be used on the toolbar

Problem/Motivation

Leaflet supports circles as markers and the drawing toolbar provided by the Leaflet Geoman drawing tools used in the leaflet form widget provide a toolbar option for this. The configuration options for the widget show it as a listed option along with a warning that it is not supported by the GeoJSON data format, and the checkbox is disabled preventing it from being ever shown.

I appreciate the label warning that circle and circle marker are not supported by GeoJSON, but I'm not sure disabling the checkbox is the right call. Circle markers are saved as POINT data in GeoJSON, the same as marker pins. For my application I prefer the circle marker to dropping a pin, so I'd prefer to keep that as an option.

I can alter the contents of leaflet maps with post-load javascript to display markers as circles, but with only markers available on the toolbar itself, such markers will be displayed as markers instead of circles while being drawn, which is not really going to work for me.

There is no real support for circles with variable radii, so I think leaving the Circle tool permanently disabled makes sense for now, at least until a better solution is found to make it usable.

Proposed resolution

Since circles technically *are* supported by GeoJSON (as POINT data) I see no reason to enforce this option being permanently disabled. I think having the checkbox unchecked by default and displaying the current warning message is sufficient and allowing them to be used with the understanding that they will display as markers on the default map unless overridden with custom javascript.

In the future we could probably provide styling options for circle markers in the style setting options and use those when displaying the maps. We could possibly add a toggle option to use either markers or circles as a toggle and provide the appropriate toolbar button and display options as per preference.

User interface changes

Enable the CircleMarker toolbar widget.