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

app/models/media_attachment.rb

@@ -16,6 +16,7 @@
 #  shortcode         :string
 #  type              :integer          default("image"), not null
 #  file_meta         :json
+#  description       :text
 #
 
 require 'mime/types'
@@ -58,6 +59,7 @@ class MediaAttachment < ApplicationRecord
   validates_attachment_size :file, less_than: 8.megabytes
 
   validates :account, presence: true
+  validates :description, length: { maximum: 140 }, if: :local?
 
   scope :attached,   -> { where.not(status_id: nil) }
   scope :unattached, -> { where(status_id: nil) }
@@ -78,6 +80,7 @@ class MediaAttachment < ApplicationRecord
     shortcode
   end
 
+  before_create :prepare_description, unless: :local?
   before_create :set_shortcode
   before_post_process :set_type_and_extension
   before_save :set_meta
@@ -136,6 +139,10 @@ class MediaAttachment < ApplicationRecord
     end
   end
 
+  def prepare_description
+    self.description = description.strip[0...140] unless description.nil?
+  end
+
   def set_type_and_extension
     self.type = VIDEO_MIME_TYPES.include?(file_content_type) ? :video : :image
     extension = appropriate_extension