Front End eslint

[AlexPikalov]

I believe it makes sense to have eslint step in CI from the FE end. It would check existing codebase for plenty of things such as unused vars, using undeclared vars, proper using var vs let vs const, code format, missed semicolons and a lot of other.

I've briefly checked current code and the linter has been able to find a lot of stuff which could (and probably should) be improved. So it might make sense to fix it before including this step into CI.