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/models/custom_emoji_spec.rb

@@ -0,0 +1,25 @@
+require 'rails_helper'
+
+RSpec.describe CustomEmoji, type: :model do
+  describe '.from_text' do
+    let!(:emojo) { Fabricate(:custom_emoji) }
+
+    subject { described_class.from_text(text, nil) }
+
+    context 'with plain text' do
+      let(:text) { 'Hello :coolcat:' }
+
+      it 'returns records used via shortcodes in text' do
+        is_expected.to include(emojo)
+      end
+    end
+
+    context 'with html' do
+      let(:text) { '<p>Hello :coolcat:</p>' }
+
+      it 'returns records used via shortcodes in text' do
+        is_expected.to include(emojo)
+      end
+    end
+  end
+end