174 zoom level display

[mradamcox]

Unification of some components that make up the map interfaces (re) used in different parts of the site. Especially

• Create reusable MousePosition controls for layer and document viewers
• Add zoom level notation to preview map and simple layer and document viewers, closes #174
• Combine SingleDocumentViewer and SingleLayerViewer into single component, SimpleVIewer
• Add Mapbox attribution to more map viewers
• Update some argument names, close #178 and #179