Asynchronous loading of geographic features

[oliverroick]

To improve the performance of client-side map rendering, the client should load features gradually and asynchronously.

This task should be coordinated with #932

Tasks

• Extend spatial unit API view with pagination. (We can think about a new “internal” API endpoint that serializes only data of spatial units that we need to render the map.)
• Adapt and add API view unit tests accordingly.
• Document pagination in the API docs.
• Remove spatial-unit serialization form template contexts and templates in all relevant map views.
• Adapt default view unit tests accordingly.
• Implement asynchronous loading on client-side for all related map views.
• Add 'loading' indicator when features are loaded from the map.