Use `GET` over `POST` where possible

we use POST because we use the spatial filtering features of WFS and allow a user to subset a layer by uploading a feature and using the WFS spatial predicates (INTERSECTS, WITHIN, etc). Sometimes this upload is bigger than can be supplied via query parameter in GET so we use POST and add the feature to the request body.

However, there are lots of requests that don't require POST, and we've had various issues with POST requests (eg. #348), so it may be a good idea to use GET except when we are doing a spatial filter.