feature request: Assign shift to user

[SjoerdHekking]

Checklist

• I checked that this feature has not been requested before
• I checked that this feature is not in the "wontfix / not planned" list
• This feature will benefit the majority of users

Problem Description / Use Case

When creating a shift manually or from a preset, it can become cluttered, especially if 10~ people need to look at it.

Proposed Solution

Allow during shift creation to assign a user to a shift, so the shift is bound to that user. This way, on the user side of things, they can toggle only their shifts to show in the calendar, or show all shifts (including shifts from other users)