Create pairing functionality

[leahriffell]

When a user submits new availability, on the BE we see if there is anyone they can pair with. If so, we create a new user pairing (and mark the user's availability block to fulfilled). If not, then sadness (user's availability block is still open).

To see if an open pairing:

• compare against any other open availabilities
• where user has not requested to be blocked
• where paired_user has not been blocked
• where paired_user's target language = user's native language
• where paired_user's native language = user's target language

If A is free 3 - 5 and B is free 2 - 10, just pick the first overlapping spot (3).