DEV: Remove JoyPixels emoji option (#12197)

- removes the option from site settings
- deletes the site setting on existing sites that have it
- marks posts using emojis as requiring a rebake

Note that the actual image files are not removed here, the plan is to
remove them in a few weeks/months (when presumably the rebaking of old
posts has been completed).

db/migrate/20210224162050_remove_emoji_one_from_emoji_set_site_setting.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class RemoveEmojiOneFromEmojiSetSiteSetting < ActiveRecord::Migration[6.0]
+  def up
+    result = execute("SELECT value FROM site_settings WHERE name='emoji_set' and value='emoji_one'")
+    return unless result.count > 0
+
+    execute "DELETE FROM site_settings where name='emoji_set' and value='emoji_one'"
+    execute "UPDATE posts SET baked_version = 0 WHERE cooked LIKE '%/images/emoji/emoji_one%'"
+  end
+
+  def down
+    # Cannot undo
+  end
+end