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

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20170913000752) do
+ActiveRecord::Schema.define(version: 20170917153509) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -89,6 +89,18 @@ ActiveRecord::Schema.define(version: 20170913000752) do
     t.index ["uri"], name: "index_conversations_on_uri", unique: true
   end
 
+  create_table "custom_emojis", force: :cascade do |t|
+    t.string "shortcode", default: "", null: false
+    t.string "domain"
+    t.string "image_file_name"
+    t.string "image_content_type"
+    t.integer "image_file_size"
+    t.datetime "image_updated_at"
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.index ["shortcode", "domain"], name: "index_custom_emojis_on_shortcode_and_domain", unique: true
+  end
+
   create_table "domain_blocks", id: :serial, force: :cascade do |t|
     t.string "domain", default: "", null: false
     t.datetime "created_at", null: false