[Glitch] Display customized mascot in web UI

Port relevant parts of 9d4541c612 to glitch-soc

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

@@ -24,6 +24,7 @@ export const searchEnabled = getMeta('search_enabled');
 export const maxChars = (initialState && initialState.max_toot_chars) || 500;
 export const invitesEnabled = getMeta('invites_enabled');
 export const version = getMeta('version');
+export const mascot = getMeta('mascot');
 export const isStaff = getMeta('is_staff');
 
 export default initialState;