Display a custom dataset for maps in Stories

[hanbyul-here]

Context

Give editors a way to add custom datasets (more specifically - static data from a file) on top of the map in Stories, so editors can annotate and highlight the area on the map.

This was technically infeasible since we only allowed one layer per map instance. But as we make progress on #712, we now have a way of displaying multiple layers on one map instance. We can leverage this improvement to offer editors a custom dataset layer.

We need to limit this dataset to a specific size, format (ex., not more than 10 MB, geojson format.), and how it can be displayed (style is prefixed. or at least prefixed with dark or light theme). Also, there is a problem with annotation. Editors would want to show what the custom data means. How should this data be displayed? Through legend? or some kinds of special annotation? Through interaction like clicking?

We would also need to replace the old map component in the scrolly map so editors can have a consolidated way of adding custom data on top of their maps. - #967

Acceptance Criteria

• Editors can add a custom dataset on top of the Map components in Stories

Related Tickets

• Replace duplicated map component #712
• Replace Mapbox component to the new Map component in ScrollyMap component #967