MapRoulette: Allow working on a specific hidden challenge

[tordans]

Description

I tend to set MapRoulette Challenges to "invisible" whenever I want to test them but also when I only want to work on them with a specific group of people.

However, it looks like there is no way to access a non-public challenge in Rapid. (Right now, 45242 is public again, to allow testing it.)

I was hoping for something like …

• When no challenge ID is given, show all public challenges
• When a specific challenge ID is given, show that one regardless of it's public state