Replace momentjs with alternative

[webketje]

Luxon looks good (also for multilang-localized dates especially) https://moment.github.io/luxon

[webketje]

Further thinking of this, given permalinks maps the date part to folder structures, there's only half or so of the formatting at the table at https://momentjs.com/docs/#/displaying/format/ that should be allowed. All the formats containing + or : or undesirable as they clash with either folder allowed chars or url-recognized chars.

99% of use cases will use either inverted ISO-like format 2022/01/01 or with some parts in full, eg archive/2022/january.
Luxon unfortunately relies on the Intl API which is not guaranteed to be available in all Node builds (regardless of version).
So the best way forward may be to provide our own limited set of formatting tokens (compatible with moment.js)