Add user rating feature

[User0332]

Feature idea: Users who complete an exchange should be able to rate each other (both the receiver and the giver) and users could hold aggregate receiver and giver ratings which are displayed on their profile.

Two objections:

• It's not worth the effort (db migration, new featureset, possibly new route, definitely new script and API endpoint, etc.)
  • Need to gauge actual request volume for this feature
• Because of the small-volume nature of book giveaways, the ratings are effectively not anonymous...