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

app/models/custom_emoji.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+# == Schema Information
+#
+# Table name: custom_emojis
+#
+#  id                 :integer          not null, primary key
+#  shortcode          :string           default(""), not null
+#  domain             :string
+#  image_file_name    :string
+#  image_content_type :string
+#  image_file_size    :integer
+#  image_updated_at   :datetime
+#  created_at         :datetime         not null
+#  updated_at         :datetime         not null
+#
+
+class CustomEmoji < ApplicationRecord
+  SHORTCODE_RE_FRAGMENT = '[a-zA-Z0-9_]{2,}'
+
+  SCAN_RE = /(?<=[^[:alnum:]:]|\n|^)
+    :(#{SHORTCODE_RE_FRAGMENT}):
+    (?=[^[:alnum:]:]|$)/x
+
+  has_attached_file :image
+
+  validates_attachment :image, content_type: { content_type: 'image/png' }, presence: true, size: { in: 0..50.kilobytes }
+  validates :shortcode, uniqueness: { scope: :domain }, format: { with: /\A#{SHORTCODE_RE_FRAGMENT}\z/ }, length: { minimum: 2 }
+
+  include Remotable
+
+  class << self
+    def from_text(text, domain)
+      return [] if text.blank?
+      shortcodes = text.scan(SCAN_RE).map(&:first)
+      where(shortcode: shortcodes, domain: domain)
+    end
+  end
+end

app/models/status.rb

@@ -131,6 +131,10 @@ class Status < ApplicationRecord
     !sensitive? && media_attachments.any?
   end
 
+  def emojis
+    CustomEmoji.from_text(text, account.domain)
+  end
+
   after_create :store_uri, if: :local?
 
   before_validation :prepare_contents, if: :local?