Support for only allowing each value in a dropdown to be selected once per form

[chrisala]

This is to support fieldcapture #2564 implement that each baseline can only be reported on once per report.

The use case is a baseline from the MERI plan can be selected from a dropdown in a table, but each table row must have a unique selection.
This has been asked to be implemented by reducing the available selections from the drop down to only include values not chosen in other rows.

For future use, it should also support the same behaviour with multi-select fields.