Move term validation into ChangeOps

NOTE: this changes constraint checking for some cases:
* sitelink constraints are now *always* checked
* label/description uniqueness becomes a "soft" constraint,
  checked only for new input.

NOTE: this also changes how sitelink constraints are enforced
in ChangeOpsMerge. This is a side effect of consolidating
the constraint handling.

Bug: 62643
Change-Id: Ibb6ee8439ba3ab0252403eb811aa48d1274d1d58
Merge "Move term validation into ChangeOps"