Update outdated NPM dependencies

This MR builds on top of !435 (merged) so it should be merged afterwards.

It updates NPM dependencies

• react-datepicker is still outdated as later version have an issue which breaks it the way we are using it, due to the migration from tether to popper.

• Nodemailer update requires Node >= 6, but this version is the up to date version, and the package altogether is no longer supported. I am not sure about the best thing to do with nodemailer. Important fact is that jessie (oldstable) still has node 4. We can revert the nodemailer update. NOT UPDATED for now

• There is a breaking change with the indent rule of eslint in 4.0.0, see https://github.com/eslint/eslint/issues/8715. I left an outdated version, but we can also choose to bump eslint and fix all the files, or wait for an alternative. Let me know what you prefer.

I tested the app and checked a bit everything (including date picker and email sending), there should not be breaking changes.