[DateTimeInput] Add "mode=week" prop

[daniel-martic-sociomantic]

• add "mode=week" that will show week number besides days and that has header week
• selected day should highlight current week
• selected week should highlight first day of the week
• we should be able to pass starting day of the week and render days accordingly (e.g. if we pass wed, days of the week should have a range from Wednesday to Tuesday etc.) #949