make it clearer that you can choose a class later

Comments in the Tavern make it obvious that players don't always realise it's possible to choose a class later. This change, plus https://github.com/HabitRPG/habitrpg/pull/3724 , will make it clearer.

(Ignore the content.coffee and habitrpg-shared.js commits. The final one undoes the changes in the first two. Sorry.)