Add option to disable emoji replacements

Fixes #647

The option is found in `/settings` (because that was easier to write it this
way) but only affects the glitch-soc front-end.

app/models/user.rb

@@ -108,7 +108,7 @@ class User < ApplicationRecord
            :reduce_motion, :system_font_ui, :noindex, :flavour, :skin, :display_media, :hide_network, :hide_followers_count,
            :expand_spoilers, :default_language, :aggregate_reblogs, :show_application,
            :advanced_layout, :use_blurhash, :use_pending_items, :trends,
-           :default_content_type,
+           :default_content_type, :system_emoji_font,
            to: :settings, prefix: :setting, allow_nil: false
 
   attr_reader :invite_code