Geospatial previewer should handle dynamic projections

[max-zilla]

The geospatial previewer expects EPSG 3857 (web mercator) which works for OSM and geotiffs in that projection, but uploading a tiff at a different projection such as EPSG 4326 does not work.

Either the previewer should cast the tiff into 3857 before upload, the vis config should store the projection so we can render dynamic layers, or another solution needs to be figured out.

A couple of options:

1. Change to EPSG 3857 in the extractor.
2. Get geoserver to serve it to you in a different projection
3. Store projection in the metadata and use that to configure openlayers.