Add ability to specify that you are pairing with a specific user for a week

[mbramson]

If a user knows that they would like to pair with a specific person the following week, there should be some way for them to specify this intention. If both users specify that they would like to pair with one another, they should be paired that following week.

Some things to think about:

• This should be something that can occur for weeks that have yet to pair.
• Since currently pairs are only made if there are no existing pairs for that specific week, this logic shouldn't get in the way of that. If there are 10 users and Users 1 and 2 specify that they would like to pair for the following week, when that week is reached, Users 1 and 2 should be paired, but the rest of the users should be paired randomly.
• If two users have specified that they'd like to pair for a specific week, when you hit repairify for that week, they should remain paired (although the others users would still potentially be shuffled)
• This should bypass the logic that prevents users from being paired with the users that they were paired with the previous week (implemented for Implement Logic Preventing Repeat Pairs #7)

This is blocked by #43 as you should be able to do this for coming weeks, which #43 will enable (without route memorization) from a UI perspective.