[BACKEND] Mongo to Postgre

[hn275]

Refactoring db functions, Mongo to Postgre

• api-lib/db/availability.js

• api-lib/db/event.js

• api-lib/db/group.js - hal

  • Update calendar strength algo

• api-lib/db/private_password.js - hal

• update test cases api-lib/db/test/**/*

• Update docs - Ben (Simple stuff about new technologies)