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/ostatus/atom_serializer_spec.rb

@@ -97,11 +97,23 @@ RSpec.describe OStatus::AtomSerializer do
 
       mentioned = element.nodes.find do |node|
         node.name == 'link' &&
-        node[:rel] == 'mentioned' &&
-        node['ostatus:object-type'] == TagManager::TYPES[:person]
+          node[:rel] == 'mentioned' &&
+          node['ostatus:object-type'] == TagManager::TYPES[:person]
       end
+
       expect(mentioned[:href]).to eq 'https://cb6e6126.ngrok.io/users/username'
     end
+
+    it 'appends link elements for emojis' do
+      Fabricate(:custom_emoji)
+
+      status  = Fabricate(:status, text: ':coolcat:')
+      element = serialize(status)
+      emoji   = element.nodes.find { |node| node.name == 'link' && node[:rel] == 'emoji' }
+
+      expect(emoji[:name]).to eq 'coolcat'
+      expect(emoji[:href]).to_not be_blank
+    end
   end
 
   describe 'render' do