Add state machine to Membership to handle membership status

[alexfinnarn]

It makes sense to play around with the concept of state machines with membership statuses.

https://github.com/aasm/aasm

Possible states:

• pending/invited
• active
• inactive
• banned/blocked

AC:

• Add aasm to Gemfile
• Add to code already in the membership model
• Add permissions based on status
• Write tests administering states

Next Steps

• It would be nice to "invite" users to a space, but that should be worked on in another issue.
• Add state machine to the Fund model.