Add a "Prepend to the current value" to the "Modify field values" action

Sorry, created the issue in the wrong project -.-

Anybody → changed the visibility of the branch 3459045-add-a-prepend-option-in-modify-field-values to hidden.

We also need to remove the blanks / spaces here, once 🐛 Remove unexpected space (" ") from Modify field values append method RTBC is agreed on by the maintainers!