Simplify canUpdateTags function in ChangeTags

Simplify canUpdateTags function in ChangeTags

This simplifies the function canUpdateTags by checking that none of the
tags are defined by an extension instead of checking that all of them
are either manually defined or not defined at all.
Doing so avoids a cache call and makes it easy to propose only relevant
tags for addition/removal to users at Special:EditTags.
This also makes the respective checks for adding/removing tags only when
necessary.

Change-Id: Iddee4d4efb109b0fccf1ece400d166147e1700fc