Investigate implications of moving to the latest version of npm

[luyangkenneth]

The team is currently standardized on npm 5.8.0 to retain the way dependency version numbers are specified in our package-lock.json (see #756 for more context)

If there are no downsides to upgrading, we should move to the latest version of npm. This will also help to unblock #808

[Chng-Zhi-Xuan]

Summary of why package-lock.json format was changed in npm@6

• npm install changes package-lock.json from exact versions to loosly versions npm/npm#20434 (comment)

I've been skimming through npm releases and there are no other significant breaking changes moving to the latest version (v6.9.0).

Although there will significant changes to package-lock.json's formatting and behaviour, the comment above provides some assurance that nothing will break.

Would like some input from @acjh, @yamgent

[acjh]

We're on npm@5 mainly because of htmlparser2.

[ang-zeyu]

Closing per #948