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

db/migrate/20170917153509_create_custom_emojis.rb

@@ -0,0 +1,13 @@
+class CreateCustomEmojis < ActiveRecord::Migration[5.1]
+  def change
+    create_table :custom_emojis do |t|
+      t.string :shortcode, null: false, default: ''
+      t.string :domain
+      t.attachment :image
+
+      t.timestamps
+    end
+
+    add_index :custom_emojis, [:shortcode, :domain], unique: true
+  end
+end