Allow users to selectively mark flooded areas about flood-map

[planemad]

This is ideally how it should work, but a lot more challenging to implement. The simplicity of the codebase and the entire system is that it just uses the street data already loaded in the browser as vector tiles. This means all streets are broken up into a segment at the edge of a tile.

Moving away from this mechanism will require a lot more spatial analysis and data validation. Something to do while doing a code rewrite.