feat(pat-date-picker): Cache the ajax call to retrieve i18n picker translations.

[thet]

Ref: scrum-2726

This implementation uses good ol' store module from @patternslib/patternslib/src/core/store - it's still handy.
However, we can spare us the supported query because the web storage is just supported in practice by all modern browsers since Firefox 2 etc.

Testing FTW, I found a case where a unsuccessful call to an i18n URL led to an error.

NOTE: in a real environment where multiple instances are
initialized at once on the same page, before the ajax call has
been completed, each instance will do an AJAX call. After that,
when navigating to other pages with other date picker instance
the cached value should be used and no more AJAX calls should be
made.

It would be hard to work around this limitation and I think it's a tradeoff we can live with, right?

[thet]

Done in: #1208