Allow members to be in multiple challenges at once and save team or not

[plinck]

Is your feature request related to a problem? Please describe.

Describe the solution you'd like
we need to get through he users being in mulitple challenges first or we open a can of worms with people not joining a team. I fact, we will need to make team participation optional since many challenges won’t have teams
Describe alternatives you've considered

Additional context
Also users should not have to be on a team and the user's team for a challenge needs to be saved so it is retained when switching

[plinck]

I added the current challenges teams to context and also changed the user DB stuff so that a user can be in multiple challenges at once. If you switch from one challenge to another to see the results, it will remember the team you were on for that challenge and properly assign it. When you change teams it will save it to current team and the default team for that challenge.

This is the first step in getting true ability for a user to track multiple challenges simultaneously

This is part of the usersInMultipleChallenges branch