Add overlay showing which shifts will be created

[mimischi]

When planning to create reoccuring shifts in the future, we now show a overlay with all times that will be created / will not be created.

ToDo

• Hook up Save and Dismiss Buttons to some actual functionality.
• Limit maximal repeating time a specific timedelta in the future.
• Add unit tests to DRF views.
• Translate all strings.

[codecov-io]

Codecov Report

Merging #474 into develop will decrease coverage by 0.46%.
The diff coverage is 73.55%.

@@             Coverage Diff             @@
##           develop     #474      +/-   ##
===========================================
- Coverage    82.91%   82.45%   -0.47%     
===========================================
  Files           40       46       +6     
  Lines         1276     1385     +109     
===========================================
+ Hits          1058     1142      +84     
- Misses         218      243      +25

Impacted Files	Coverage Δ
clock/shifts/views.py	90.9% <ø> (+4.28%)	⬆️
clock/shifts/urls.py	100% <ø> (ø)	⬆️
clock/shifts/forms.py	96.36% <100%> (+0.08%)	⬆️
clock/shifts/utils.py	79.8% <100%> (+8.57%)	⬆️
clock/shifts/api/serializers.py	100% <100%> (ø)
clock/contracts/api/serializers.py	100% <100%> (ø)
clock/shifts/api/urls.py	100% <100%> (ø)
clock/contracts/api/urls.py	100% <100%> (ø)
clock/shifts/api/views.py	40.81% <40.81%> (ø)
clock/contracts/api/views.py	72.72% <72.72%> (ø)
... and 6 more

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