This commit moves the calendar date and time picker shown in
the local dates modal into a core component that can be reused
in other places. Also add system specs to make sure there isn't
any breakages with this feature, and a section to the styleguide.
In #15474 we introduced dedicated support for date ranges. As part of that
change we added a fallback of "magic" date ranges, which treats dates in
any paragraph with exactly two dates as a range. There were discussions
about migrating all such paragraphs to use the new date range element, but
it was ultimately decided against.
This change removes the fallback and, as a bonus, adds support for multiple
date ranges in the same paragraph.
