Implementation notes

A few notes from our planning docs that we may want to split out into separate issues.

### Fundamental Needs
- Spatial index - allow features to be found when tapping on the map or intersecting with a selection bounding box / radius (likely built into the map renderer)
- Tag (inverted) index - allow features to be found based on included tags (e.g. show features potentially related to a survey)
- Native OSM element representation - store standard OSM attributes
- Spatial element representation - renderer- / spatial-friendly geometry (GeoJSON?)
- Renderer-friendly representation - GeoJSON? / MVT?
- Mechanism for updating spatial / renderer-friendly representations
- Trie - for grouping presets into a tree of options vs. needing to select from a long list

### Reusing duplicate answers across observations when needed
When a feature needs to be created (e.g. school POI) in order for an observation to be attached to it (i.e. 2 observations will be created), we can merge presets (school + questions) in the observation UI and allow them both to be created simultaneously (as though it were creating a single thing), tracking which preset responses are intended to be associated with each observation (feature, response).