Add ability to choose between MM/dd and dd/MM date formats

This set of changes addresses issue #4355, https://github.com/HabitRPG/habitrpg/issues/4355. It adds a "dateFormat" preference to the user model, adds a form on the settings page to choose the format, and changes date displays to use the dateFormat preference. Apologies for the mess of commits, I'm still learning git and went through more than one iteration of how to do this.