Add Event activity-type support (#12637)

This adds support for Event AP type in Mastodon. Events are converted
into toots by taking their title (AS name) and their URL (AP ID). Event
picture is also brought in if available.

Testable by fetching event content from https://test.mobilizon.org

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

spec/services/activitypub/fetch_remote_status_service_spec.rb

@@ -104,6 +104,26 @@ RSpec.describe ActivityPub::FetchRemoteStatusService, type: :service do
       end
     end
 
+    context 'with Event object' do
+      let(:object) do
+        {
+          '@context': 'https://www.w3.org/ns/activitystreams',
+          id: "https://#{valid_domain}/@foo/1234",
+          type: 'Event',
+          name: "Let's change the world",
+          attributedTo: ActivityPub::TagManager.instance.uri_for(sender)
+        }
+      end
+
+      it 'creates status' do
+        status = sender.statuses.first
+
+        expect(status).to_not be_nil
+        expect(status.url).to eq "https://#{valid_domain}/@foo/1234"
+        expect(strip_tags(status.text)).to eq "Let's change the world https://#{valid_domain}/@foo/1234"
+      end
+    end
+
     context 'with wrong id' do
       let(:note) do
         {