add mapbox-gl dependencies

[Rineee]

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Comment/Question:
I might be totally wrong here, but from what I understood, we need to add mapbox-gl dependency in all projects using A4, since we are using mapboxgl-leaflet in adhocracy4/maps/static/a4maps/a4maps_common.js and that again is added to index.js. So wouldnt it make more sense to add that dependency in A4? @phillimorland @khamui
Screenshot?

[fuzzylogic2000]

@goapunk @phillimorland Didn't we just add that to some other project? Or was it a4? I saw it in some PR...

[philli-m]

@fuzzylogic2000 #1152