Store custom layers as FeatureCollection

[zod]

Replace custom format with GeoJSON Features which are already used for layer configuration. This allows storing also MVT or GeoJSON layers.

Currently custom layers are stored in localStorage in a custom format which supports only TMS, OverpassAPI and OpenStreetMapNotesAPI(?). For #775 support for additional formats is required. Instead of extending the custom format this reuses the existing handling from LayerConfig which uses JOSM's map format serialized to GeoJSON.

Existing localStorage are converted to the new format.