Removing redundant method in UserPickerType
Adding AbstractPickerDataTransformer::setRestriction() method, adding test
Fixing typo
Updating AbstractPickerDataTransformer so it is restriction-aware and returns the correct return type
Minor alterations to AbstractPickerTypeTest
Adding exception message in AbstractPickerDataTransformer
Adding support for single select restriction in AbstractPickerType
Removing unnecessary methods from ClientPickerType
Updating ProjectFormType to use client picker correctly, updating tests