Quality-time doesn't use browser locale for first day of week

[fniessink]

Describe the bug
Quality-time doesn't use browser locale for first day of the week, except for a work-around implemented for Dutch in components/frontend/src/locale.js.

Expected behaviour
Quality-time should use the browser locale to determine the first day of the week in date pickers.

Additional context
The date time library used, dayjs, has no generic method to automatically determine the user's locale, see iamkun/dayjs#732.