Improvements to asset layer user interaction

This commit allows asset layers to be added to the map that have more
complex behaviours than simply 'click a marker'.

 - Layers can be non-interactive and/or always visible
    Previously an asset layer would only be shown when its associated
    category was chosen from the dropdown. You can now add layers which
    are always visible, as well as ignoring user input. Useful for, e.g.
    showing an LLPG overlay on the map.
 - Stylemaps can be provided for individual layers
    This means you can, for example, have custom rules for styling
    features based on their attributes or other complex rules.
 - Functions can be used for the feature/report field mapping
    Instead of a one-to-one mapping between feature attributes and
    fields on the new report form, you can provide a callback function
    which can provide a calculated value to the report form.

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