Introduce bp_group_documents_dir_url

* Audit usage of BP_GROUP_DOCUMENTS_DIR constant
Clean up some long lines
Improve get_icon logic

* Condense icons array
* Offload the extension dirty work to wp_check_filetype
Introduce bp_group_documents_get_mime_type()

WP already has an extensive mime type list. Use that instead of reinventing the wheel.
* Deprecate custom mime_content_type