Custom emoji (#4988)

* Custom emoji

- In OStatus: `<link rel="emoji" name="coolcat" href="http://..." />`
- In ActivityPub: `{ type: "Emoji", name: ":coolcat:", href: "http://..." }`
- In REST API: Status object includes `emojis` array (`shortcode`, `url`)
- Domain blocks with reject media stop emojis
- Emoji file up to 50KB
- Web UI handles custom emojis
- Static pages render custom emojis as `<img />` tags

Side effects:

- Undo #4500 optimization, as I needed to modify it to restore
  shortcode handling in emojify()
- Formatter#plaintext should now make sure stripped out line-breaks
  and paragraphs are replaced with newlines

* Fix emoji at the start not being converted

spec/lib/activitypub/activity/create_spec.rb

@@ -17,6 +17,7 @@ RSpec.describe ActivityPub::Activity::Create do
 
   before do
     stub_request(:get, 'http://example.com/attachment.png').to_return(request_fixture('avatar.txt'))
+    stub_request(:get, 'http://example.com/emoji.png').to_return(body: attachment_fixture('emojo.png'))
   end
 
   describe '#perform' do
@@ -217,5 +218,29 @@ RSpec.describe ActivityPub::Activity::Create do
         expect(status.tags.map(&:name)).to include('test')
       end
     end
+
+    context 'with emojis' do
+      let(:object_json) do
+        {
+          id: 'bar',
+          type: 'Note',
+          content: 'Lorem ipsum :tinking:',
+          tag: [
+            {
+              type: 'Emoji',
+              href: 'http://example.com/emoji.png',
+              name: 'tinking',
+            },
+          ],
+        }
+      end
+
+      it 'creates status' do
+        status = sender.statuses.first
+
+        expect(status).to_not be_nil
+        expect(status.emojis.map(&:shortcode)).to include('tinking')
+      end
+    end
   end
 end