Remove rails_admin_histories table

This was a leftover from rails_admin which was removed in #7440.

closes #7597

Changelog.md

@@ -11,6 +11,7 @@
 * Move enable/disable notification icon [#7592](https://github.com/diaspora/diaspora/pull/7592)
 * Use Bootstrap 3 progress-bar for polls [#7600](https://github.com/diaspora/diaspora/pull/7600)
 * Enable frozen string literals [#7595](https://github.com/diaspora/diaspora/pull/7595)
+* Remove `rails_admin_histories` table [#7597](https://github.com/diaspora/diaspora/pull/7597)
 
 ## Bug fixes
 * Fix displaying polls with long answers [#7579](https://github.com/diaspora/diaspora/pull/7579)

db/migrate/20170827222357_remove_rails_admin_histories.rb

+# frozen_string_literal: true
+
+class RemoveRailsAdminHistories < ActiveRecord::Migration[5.1]
+  def up
+    drop_table :rails_admin_histories
+  end
+
+  def down
+    create_table :rails_admin_histories do |t|
+      t.text     :message
+      t.string   :username
+      t.integer  :item
+      t.string   :table
+      t.integer  :month,      limit: 2
+      t.integer  :year,       limit: 8
+      t.datetime :created_at,           null: false
+      t.datetime :updated_at,           null: false
+    end
+
+    add_index :rails_admin_histories, %i[item table month year], name:   :index_rails_admin_histories,
+                                                                 length: {table: 188}
+  end
+end