Consider replacing MomentJS with date-fns

[FrancescoCioria]

requirements

MomentJS has caused us some trouble in the past because of its mutability (still, we've been bug-free for a long time now) but most of all, is huge.
We really need MomentJS over native Date only for its improved validation and format functions.

date-fns may be a great substitute as it is lighter than MomentJS but should have the same features.

specs

Let's investigate date-fns further and, if worth the effort, let's replace MomentJS with it :)

misc

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