Improve backend API

[pantierra]

This is a follow up to #1780.

• [ ] Rely on Flask 1.1 instead of unmaintained FlaskRestful.
• [ ] RESTify all endpoints
• [ ] Improve performance: Profiling space & time complexity
• [ ] Verify Data units (metrics -area, count): Make sure the data from different endpoints return numbers in the right units. Example, area measurement is expected in sq.kms, but the endpoint returns data in a different unit

More detailed info and plans in the working document of the API changes.

[willemarcel]

Maybe we should rename /api/v2/projects/{project_id}/tasks/actions/unlock-after-mapping/{task_id}/ to set-as-mapped, unlock seems like you're going back to ready status not advancing to mapped