solr-spatial-field search backend errors when searching across the anti-meridian

# Context
Tested using CKAN 2.7.4 and solr 6.5 with the JTS 1.14 plugin and the solr-spatial-field configured in the schema.xml.

# Issue

When using the solr-spatial-field search backend, and querying across the anti-meridian I.E https://demo.ckan.org/dataset?ext_bbox=177,-38,184,-37 then the query generated for solr will return a 400 http response.

The problem is that the `ENVELOPE` part of the query has a `maxx` value that is greater than 180. when solr expects the value to be between -180 and 180.

# Solution

In the `_params_for_solr_spatial_field_search` method, we could move the value into the expected bounds.

```py
def _params_for_solr_spatial_field_search(self, bbox, search_params):
'''
This will add an fq filter with the form:

+spatial_geom:"Intersects(ENVELOPE({minx}, {miny}, {maxx}, {maxy}))

'''
if bbox['maxx'] > 180:
bbox['maxx'] = -180 + (bbox['maxx'] - 180)

search_params['fq_list'] = search_params.get('fq_list', [])
search_params['fq_list'].append('+spatial_geom:"Intersects(ENVELOPE({minx}, {maxx}, {maxy}, {miny}))"'
.format(minx=bbox['minx'], miny=bbox['miny'], maxx=bbox['maxx'], maxy=bbox['maxy']))
```

Start with the `_params_for_solr_spatial_field_search` method and inspect how the bounding box becomes the Solr `ENVELOPE` query. Reproduce the demo query `ext_bbox=177,-38,184,-37` against the solr-spatial-field configuration in `schema.xml`; done means the generated query keeps longitude values within Solr's -180 to 180 bounds and no longer returns HTTP 400.

Written by the indexing model from the issue text.