Rework URL patterns

[rolandgeider]

Rework the URL patterns, specifically

• make URLs consistent, at the moment there are some that end with a slash, others that don't
• make them more hackable, like the calendar ones: /calendar/2015/07/10 and /calendar/2015/07 both work and do the expected thing. This could be also done for workout lists, etc.
• (potentially a lot of work) add the username as a prefix to most URLs, like /the_user/workout/123, similarly to what github does. This makes more sense if there is a user profile page as well.