Conference: values-* fields in the original submission become value-* in the blind submission

[haroldrubio]

When the blind submission invitation gets created, fields that accept more than one value originally only accept a single value in the blind submission since there is no check to do so:

https://github.com/openreview/openreview-py/blob/80d8607be94ae430f744d86eff176d0da07f5304/openreview/conference/invitation.py#L100-L121