Feature: #108463 - User-specific configuration of date-timepicker's first day of a week 

See forge#108463

Description 

Previously, the date-time picker used the selected locale of a backend user.

However, certain locale configurations might rather be a user preference. For example, users may prefer an english backend but want to use a weekday start on "Monday" instead of "Sunday" due to their cultural habits.

Thus, the "first day of a week" has been decoupled from the locale selection and can be configured on a per-user setting. By default, it still inherits the locale's default' setting, if not changed (for example, english=sunday and german=monday).

Inside the user settings and tab panel Backend appearance, a new dropdown First day of week in calendar popups appears.

The setting is stored in both the persisted be_users.uc preference blob, as well as on the JavaScript-persistence side.

Impact 

Editors can now choose the first day of a week as a user preference, independent from locale selection.