Geoa contrib module using Geoalchemy2 elements with SRID support

I tweaked `geoa` to support `Geoalchemy2`s `WKBElements` rather than using `Shapely.shape`. This allows it to be dropped right into support existing workflows. It also allows it to support SRIDs and transform between the column's native type and web display (4326?) and back.

Currently it should not actually work as I do not know how to get a `db.session` in a [WTForms field](https://github.com/abkfenris/flask-admin/compare/geoa-wkbelement?expand=1#diff-c1b126538da18c02c0bdd67bdf487cd8R50).

Also the in list previews are not working.

@singingwolfboy