implement backend for skilled impact project reviews

[sumeet-bansal]

database schema changes:

add PROJECT_REVIEWS table
- id
- review
- likely_to_recommend
- supporter_communication
- provided_value
- supporter_user_id
- conservationist_user_id
- skilled_impact_request_id

add ratings to USERS table (add integer columns 'total_stars' and 'total_reviews')

API changes:

POST /api/reviews
- new route
- posts the project review

GET /api/users/:id
- modify existing route
- add average rating

GET /api/contributors?skills=XXX&distance=XXX&…
- modify existing route
- add average rating