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

spec/models/media_attachment_spec.rb

@@ -17,7 +17,6 @@ RSpec.describe MediaAttachment, type: :model do
       expect(media.file.meta["original"]["height"]).to eq 128
       expect(media.file.meta["original"]["aspect"]).to eq 1.0
     end
-
   end
 
   describe 'non-animated gif non-conversion' do
@@ -50,4 +49,12 @@ RSpec.describe MediaAttachment, type: :model do
       expect(media.file.meta["small"]["aspect"]).to eq 400.0/267
     end
   end
+
+  describe 'descriptions for remote attachments' do
+    it 'are cut off at 140 characters' do
+      media = Fabricate(:media_attachment, description: 'foo' * 100, remote_url: 'http://example.com/blah.jpg')
+
+      expect(media.description.size).to be <= 140
+    end
+  end
 end