Monica: Add a setting to change the first day of the week

[asbiin]

Sunday is not always the first day of the week, it depends on the locale.

This will be usefull in

• a calendar
• setting a birthday date for a contact, which uses vuejs-datepicker component, see https://github.com/charliekassel/vuejs-datepicker

[TheGP]

Sunday is the end of the week