Add index to document type

This will speed up the document type search in Feedex.

For: https://trello.com/c/IB9P5EyE/266-introduce-new-browse-options-for-feedex

db/migrate/20180108153838_add_index_to_document_type.rb

@@ -0,0 +1,5 @@
+class AddIndexToDocumentType < ActiveRecord::Migration[5.0]
+  def change
+    add_index :content_items, :document_type
+  end
+end

db/structure.sql

@@ -355,6 +355,13 @@ CREATE INDEX index_anonymous_contacts_on_created_at_and_path ON anonymous_contac
 CREATE INDEX index_anonymous_contacts_on_path ON anonymous_contacts USING btree (path varchar_pattern_ops);
 
 
+--
+-- Name: index_content_items_on_document_type; Type: INDEX; Schema: public; Owner: -; Tablespace: 
+--
+
+CREATE INDEX index_content_items_on_document_type ON content_items USING btree (document_type);
+
+
 --
 -- Name: index_content_items_organisations_unique; Type: INDEX; Schema: public; Owner: -; Tablespace: 
 --
@@ -411,6 +418,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20160822145924'),
 ('20160826105129'),
 ('20171204124407'),
-('20171204155340');
+('20171204155340'),
+('20180108153838');