[Go] Split frontend into frontend and backend

[sergey-shambir]

Currently, frontend written with Node.js and serves HTML page for client browser. It's better to separate domain logic into new component - 'backend', written in Go. The new component should have JSON REST API, which serves frontend. Frontend should only serve HTML pages without any domain logic.

• [x] create Go library restapi with common REST API service code shared between builder and backend components
• [x] refactor builder component to use restapi library, run builder tests (Python script)
• [x] create backend component which re-implements student use cases
• [x] write tests for the backend - in Python, just like tests for the builder
• [x] re-implement buildlistener.js in backend_service
• [x] refactor frontend to use backend internally - delete SQL connector, Repository, rewrite backendapi.js to use backend_service instead ofbuilder_service`, remove RabbitMQ integration, rewrite request handlers

[sergey-shambir]

Refactoring mostly finished, frontend now works with backend, changes pushed to master branch

[sergey-shambir]

I've implemented buildlistener.go which listens for BuildFinished event, takes build report and updated commit and solution tables. So refactoring finished.