Fix/shift create

[pablo-schmeiser]

Fixing the following bugs:

• Fixing incorrect formatting of date data in the day_calendar view, making it impossible to create shifts from this view
• Adding form validation to check whether end is after start in shifts, which did not produce error messages before but just soft locked users in the shift creation form
• Overwriting z-index of modal backdrop because of an issue with Bootstrap in safari on IOS soft locking users by rendering the backdrop in front of the modal