Introduce CI/CD

[nksazonov]

Part of #11.

Improvements:

General:

1. incorporate a method of storing minor/patch changes for Vault, YellowClearing and other smart contracts -> issues labeling system

Per commit:

1. commit message linting
2. add 'eslint checks + formatting' step

Per PR:

1. add 'compilation' step
2. add 'test' step
3. add 'gas benchmarks' step
4. add 'other script checks' (e.g. go bindings generation) step
5. write a doc about steps of checking SC for bugs and unintended behaviour