Mass Deleter: Improve date formatting

[marcustyphoon]

User-facing changes

Improves the visual presentation of the date in the Mass Deleter confirmation dialogue, removing d/m/y vs m/d/y ambiguity and the unnecessary seconds display and including the timezone.

before	after

Technical explanation

Feeds the configured page locale to Intl.DateTimeFormat even though the UI text is of course English. I think this probably makes sense...?

Issues this closes

n/a