Display customized mascot in web UI and fix admin form for it (#8964)

Follow-up to #8766

app/javascript/mastodon/initial_state.js

@@ -14,5 +14,6 @@ export const me = getMeta('me');
 export const searchEnabled = getMeta('search_enabled');
 export const invitesEnabled = getMeta('invites_enabled');
 export const version = getMeta('version');
+export const mascot = getMeta('mascot');
 
 export default initialState;