Round lon-lat and restrict to single shape

This adds two minor features required for the overall task of adding geometry to Monitoring Facilities:

1. the lon-lat values generated by the leaflet map are rounded to 5 decimal places before being added to the geojson text box and are consequently saved to the metadata record with 5 decimal places. This provides a precision of around 1m.

2. when editing a geometry feature on the leaflet map, the 'save' button has been removed to prevent the user from adding multiple features. This had to be done via css in the end (https://stackoverflow.com/questions/51360283/how-to-remove-save-option-from-leaflet-draw-api-delete-button) as there is not option in the leaflet-draw plugin to do it (https://leaflet.github.io/Leaflet.draw/docs/leaflet-draw-latest.html).

Closes EMC-44