Check uniqueness only for edited language

This optimizes uniqueness checks to only apply to labels and
descriptions that were not already present before a given edit.

This reduces database load, but more importantly, this means that
if an item has a label conflict in english, the german language
label can still be edited.

Change-Id: I7d72077af67b6698aec8e28f56e22e6fb4e4bfd2
Merge "Check uniqueness only for edited language"