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/javascript/flavours/glitch/util/initial_state.js

@@ -31,5 +31,6 @@ export const defaultContentType = getMeta('default_content_type');
 export const forceSingleColumn = getMeta('advanced_layout') === false;
 export const useBlurhash = getMeta('use_blurhash');
 export const usePendingItems = getMeta('use_pending_items');
+export const useSystemEmojiFont = getMeta('system_emoji_font');
 
 export default initialState;