[Feat] Update to use dmc 0.15.1 Part 2

[antonymilne]

Description

Commits that should have been part of #924 but weren't for technical reasons... The most important of these is the first, which ensures that you can still use vizro offline.

FYI @AnnMarieW

Move mantine_dates.css into repo; remove external_stylesheet; delete datepicker.css
Make layout children a list
Add changelog

Screenshot

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