Add ability to specify alternative text for media attachments (#5123)

* Fix #117 - Add ability to specify alternative text for media attachments

- POST /api/v1/media accepts `description` straight away
- PUT /api/v1/media/:id to update `description` (only for unattached ones)
- Serialized as `name` of Document object in ActivityPub
- Uploads form adjusted for better performance and description input

* Add tests

* Change undo button blend mode to difference

db/migrate/20170927215609_add_description_to_media_attachments.rb

@@ -0,0 +1,5 @@
+class AddDescriptionToMediaAttachments < ActiveRecord::Migration[5.1]
+  def change
+    add_column :media_attachments, :description, :text
+  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: 20170924022025) do
+ActiveRecord::Schema.define(version: 20170927215609) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -161,6 +161,7 @@ ActiveRecord::Schema.define(version: 20170924022025) do
     t.string "shortcode"
     t.integer "type", default: 0, null: false
     t.json "file_meta"
+    t.text "description"
     t.index ["account_id"], name: "index_media_attachments_on_account_id"
     t.index ["shortcode"], name: "index_media_attachments_on_shortcode", unique: true
     t.index ["status_id"], name: "index_media_attachments_on_status_id"