Mapillary integration MVP

Breaking out from #1737

Let's work on a first stab at Mapillary integration. The goal here is to enable anyone to create MapRoulette challenges based on sepecifc Mapillary detections classes. Let's start with a limited number of detections:

• ---manholes---
• crosswalks
• benches

The main problems to solve are:

• Connecting the Mapillary API to retrieve detections
• decode and filter the data Mapillary API returns (vector tiles)
• Let the user define an area of interest for the challenge (we may need to tackle #1797 first)
• [optional but really nice to have] de-duplicate against existing OSM using Overpass call
• UX for challenge creation rework

A rough description of the workflow would be:

1. User creates new challenge
2. Fourth option for task source would be added: "I want to use Mapillary detections"
3. User would be able to select detection type from a dropdown (see options above, to be expanded later)
4. User would define an area of interest
5. [optional -- at this point we would query the API and provide an estimated number of tasks that the challenge would contain]
6. [optional but really nice to have] we would query the overpass API and remove features that likely (based on proximity) are already mapped in OSM
7. User would complete the challenge creation using the existing wizard options. #844 would also be a prerequisite.

contact person from Mapillary side: @cbeddow

Start by reviewing the existing challenge-creation wizard and task-source flow; no implementation files or tests are named. Map the prerequisites in #1797 and #844, then define the scope for Mapillary API retrieval, vector-tile decoding and filtering, detection selection, and area-of-interest handling before considering optional Overpass deduplication.