Support for automatic "Advanced Search" forms.

This feature adds a new optional field-level flag, `make_searchable: True`. When this field exists on at least one field an advanced search page will be created. Currently it adds a link under the search bar:


This page will render all fields in the schema marked `make_searchable`, re-using the existing form snippets to render them.


If a date pair exists (`start_date` and `end_date` by default), a date range selector will be rendered instead using bootstraprap-daterangepicker (with small fixes to make it compatible with all versions of CKAN bootstrap and icons):


Hitting search generates a query and redirects to the normal search page. If you got to the advanced search page from a type-specific page (ex: /my-schema) it'll redirect there. Since this tool is just a query builder, normal search plugins, sort, faceting etc all work as expected.


The one gotcha is that date fields need to end in `_date` when using the default CKAN solr schema, since this will cause them to index as TrieDateField.