Handle character limit in initial state in frontend.

app/javascript/mastodon/initial_state.js

@@ -11,5 +11,6 @@ export const boostModal = getMeta('boost_modal');
 export const deleteModal = getMeta('delete_modal');
 export const me = getMeta('me');
 export const searchEnabled = getMeta('search_enabled');
+export const maxChars = getMeta('max_toot_chars') || 500;
 
 export default initialState;