Allow only one ALIAS per bucket

As of now it is possible to create multiple ALIASes per bucket. This can lead to poor code readability as the same bucket can be referred to with different names. Hence, allow only one alias per bucket.

As we allow intermediate rows to be added and removed, this cannot be checked on a per-row basis, but just before the user clicks on Continue/Next to go to the Edit Javascript screen. Hence, show the LoV with all buckets listed, but do the one-alias-per-bucket check just before Proceed.