Use secure token to create attachment links

The link is public but protected by a secure token.

This extends the activesupport Attachment model to automatically
generate a token

db/migrate/20200412214304_add_token_to_attachments.rb

@@ -0,0 +1,6 @@
+class AddTokenToAttachments < ActiveRecord::Migration[6.0]
+  def change
+    add_column :active_storage_attachments, :token, :string
+    add_index :active_storage_attachments, :token, unique: true
+  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: 2020_04_12_165834) do
+ActiveRecord::Schema.define(version: 2020_04_12_214304) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -21,8 +21,10 @@ ActiveRecord::Schema.define(version: 2020_04_12_165834) do
     t.bigint "record_id", null: false
     t.bigint "blob_id", null: false
     t.datetime "created_at", null: false
+    t.string "token"
     t.index ["blob_id"], name: "index_active_storage_attachments_on_blob_id"
     t.index ["record_type", "record_id", "name", "blob_id"], name: "index_active_storage_attachments_uniqueness", unique: true
+    t.index ["token"], name: "index_active_storage_attachments_on_token", unique: true
   end
 
   create_table "active_storage_blobs", force: :cascade do |t|