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.

config/locales/simple_form.en.yml

@@ -124,6 +124,7 @@ en:
         setting_reduce_motion: Reduce motion in animations
         setting_show_application: Disclose application used to send toots
         setting_skin: Skin
+        setting_system_emoji_font: Use system's default font for emojis (applies to Glitch flavour only)
         setting_system_font_ui: Use system's default font
         setting_theme: Site theme
         setting_trends: Show today's trends

config/settings.yml

@@ -29,6 +29,7 @@ defaults: &defaults
   reduce_motion: false
   show_application: false
   system_font_ui: false
+  system_emoji_font: false
   noindex: false
   hide_followers_count: false
   enable_keybase: true