Document Type Search

**Document Type Search**

This PR add a "Document Type" filter section to the document search page (document_list.html).  The section is placed when the regular "Type" filter section is on the layers_list and global search pages.  This PR doesn't fully replace the need for downstream GeoNode forks to customize the searching/browsing experience.  However, it should benefit basic GeoNode deployments that will host a large number of photos or static maps (pdfs).

Documents are assigned to 5 "doc_types" based on looking up their extensions in DOCUMENT_TYPE_MAP in the settings.py:
1.  Text
2.  Image
3.  Presentation
4.  Archive (.zip, .gz)
5.  Other

If the extension is not found in settings.DOCUMENT_TYPE_MAP then "doc_type" is set as "other".  Let me know if I should add other doc_types or extensions to settings.py.  Or reorganize them in some way.

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Other Notes:
+This does not adjust the core type field.  It is still set to 'document' for documents'.
+This required refractoring the facets function in geonode/base/templatetags/base_tags.py.
+This required abstracting the search/_search_content.html template to require a header and filter variable to be set.
is the ?type__in=document needed? The documents api endpoint should already return only documents