Implement Logic Preventing Repeat Pairs

[mbramson]

When pairs are made, prevent pairs from forming where that pair occurred the week before.

Some thinking could go into this, but I'm thinking a 3 person pair of person A, B, and C is distinct from a pair of A and B.

Obviously a pair of A and B is the same as a pair of B and A. Same thing goes for a three person pair of A, B, and C and a three person pair of C, B, and A.

If #34 is implemented before this is complete, we shouldn't disallow users that were paired last week from specifying that they'd like to pair the following week too if they both select one another.