Based on issues identified in https://meta.discourse.org/t/improved-bookmarks-with-reminders/144542/20
* Implement the resolvedTimezone() function on the user model where we return the user's timezone if it has been set, or we guess it using moment and save it to the user using an update call if it has not yet been set. This covers the cases of users who do not log out/in often who will not get their timezone set via login. This also makes sure the guess + save is done in a non-obtrusive way not on every page -- only when it is needed.
* Before if a user's timezone was blank when they visited their profile page we were autofilling the dropdown with the guessed timezone from moment. However this was confusing as it would appear you have that timezone saved in the DB when you really didn't. Now we do not autofill the dropdown and added a button to automatically guess the current timezone to make everything more explicit.
