Fix #416 - Generate random unique 14-byte (19 characters) shortcodes

for local attachments, use them in URLs. Check status privacy
before redirecting to actual file.

db/migrate/20170105224407_add_shortcode_to_media_attachments.rb

@@ -0,0 +1,14 @@
+class AddShortcodeToMediaAttachments < ActiveRecord::Migration[5.0]
+  def up
+    add_column :media_attachments, :shortcode, :string, null: true, default: nil
+    add_index :media_attachments, :shortcode, unique: true
+
+    # Migrate old links
+    MediaAttachment.local.update_all('shortcode = id')
+  end
+
+  def down
+  	remove_index :media_attachments, :shortcode
+  	remove_column :media_attachments, :shortcode
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20161222204147) do
+ActiveRecord::Schema.define(version: 20170105224407) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -95,6 +95,8 @@ ActiveRecord::Schema.define(version: 20161222204147) do
     t.integer  "account_id"
     t.datetime "created_at",                     null: false
     t.datetime "updated_at",                     null: false
+    t.string   "shortcode"
+    t.index ["shortcode"], name: "index_media_attachments_on_shortcode", unique: true, using: :btree
     t.index ["status_id"], name: "index_media_attachments_on_status_id", using: :btree
   end