Migrate fixed vocabulary term reference fields as translatable to conform to data expectations when nodes get merged

Problem/Motivation

While working on #3035392: Migrate vocabulary translations and taxonomy term references for Drupal 7 node translations it was discovered that the term reference for the vocabulary fixed field wasn't working. What it was doing was migrating the term reference used the last migrated translation to all the translations of that node. For example, node/1 had a term reference of 'X' and the Icelandic translation did not have a term reference. When that got migrated both node/1 and is/node/1 had a term reference of NULL.

After working with Gabor I finally saw the problem, it was the setting 'Users may translate this field' on admin/structure/types/manage/article/fields/node.article.vocab_fixed. When FALSE, we get the behavior described above and when TRUE we get the behavior we want.

This issue is to correct that problem. The original issue is #2979970: Migrate D7 vocabulary language settings which has excellent graphics by masipila in the IS. Highly recommend you take a look.

Proposed resolution

Change the vocab fixed language setting migration from:
D8 content type field settings for the term reference 'Users may translate this field': FALSE

To;
D8 content type field settings for the term reference 'Users may translate this field': TRUE

Remaining tasks

Patch
Review
Commit