Better eslint rules

[chabou]

ESLint rules should be challenged.
Detecting unused import should be added for example

[nicobarray]

EDIT: I'll use this comment to list all eslint rules we should/must add in the future to keep track of what is done. Further comments can discuss the rationale to enable or disable any rule.

Rules to enable:

• no-unused-vars. Resolved by 5466f66.
• Sort/Order imports.