Allowing the user to choose shifts directly or assigning them to an offspring

[chargio]

Allow the admin to choose between:

• Customers choosing shifts themselves (no offspring)
• Offspring being assigned to shifts (no parents)
• Both

And configuring it in the environment, the configuration does not need to be dynamic, but has to be documented

[chargio]

This should be done changing the way assignments are done. Currently they are related to offspring, so either a new type of offspring (self) is used, or a change in assignments is done.