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/formatter_spec.rb

@@ -223,6 +223,45 @@ RSpec.describe Formatter do
 
         include_examples 'encode and link URLs'
       end
+
+      context 'with custom_emojify option' do
+        let!(:emoji) { Fabricate(:custom_emoji) }
+        let(:status) { Fabricate(:status, account: local_account, text: text) }
+
+        subject { Formatter.instance.format(status, custom_emojify: true) }
+
+        context 'with emoji at the start' do
+          let(:text) { ':coolcat: Beep boop' }
+
+          it 'converts shortcode to image tag' do
+            is_expected.to match(/<p><img draggable="false" class="emojione" alt=":coolcat:"/)
+          end
+        end
+
+        context 'with emoji in the middle' do
+          let(:text) { 'Beep :coolcat: boop' }
+
+          it 'converts shortcode to image tag' do
+            is_expected.to match(/Beep <img draggable="false" class="emojione" alt=":coolcat:"/)
+          end
+        end
+
+        context 'with concatenated emoji' do
+          let(:text) { ':coolcat::coolcat:' }
+
+          it 'does not touch the shortcodes' do
+            is_expected.to match(/:coolcat::coolcat:/)
+          end
+        end
+
+        context 'with emoji at the end' do
+          let(:text) { 'Beep boop :coolcat:' }
+
+          it 'converts shortcode to image tag' do
+            is_expected.to match(/boop <img draggable="false" class="emojione" alt=":coolcat:"/)
+          end
+        end
+      end
     end
 
     context 'with remote status' do
@@ -231,6 +270,45 @@ RSpec.describe Formatter do
       it 'reformats' do
         is_expected.to eq 'Beep boop'
       end
+
+      context 'with custom_emojify option' do
+        let!(:emoji) { Fabricate(:custom_emoji, domain: remote_account.domain) }
+        let(:status) { Fabricate(:status, account: remote_account, text: text) }
+
+        subject { Formatter.instance.format(status, custom_emojify: true) }
+
+        context 'with emoji at the start' do
+          let(:text) { '<p>:coolcat: Beep boop<br />' }
+
+          it 'converts shortcode to image tag' do
+            is_expected.to match(/<p><img draggable="false" class="emojione" alt=":coolcat:"/)
+          end
+        end
+
+        context 'with emoji in the middle' do
+          let(:text) { '<p>Beep :coolcat: boop</p>' }
+
+          it 'converts shortcode to image tag' do
+            is_expected.to match(/Beep <img draggable="false" class="emojione" alt=":coolcat:"/)
+          end
+        end
+
+        context 'with concatenated emoji' do
+          let(:text) { '<p>:coolcat::coolcat:</p>' }
+
+          it 'does not touch the shortcodes' do
+            is_expected.to match(/<p>:coolcat::coolcat:<\/p>/)
+          end
+        end
+
+        context 'with emoji at the end' do
+          let(:text) { '<p>Beep boop<br />:coolcat:</p>' }
+
+          it 'converts shortcode to image tag' do
+            is_expected.to match(/<br><img draggable="false" class="emojione" alt=":coolcat:"/)
+          end
+        end
+      end
     end
   end