D7 taxonomy term fields are not migrated with allowed vocabularies

Problem/Motivation

On migration from D7, the taxonomy term fields get migrated fine, but they are set to allow all vocabularies, even though that wasn't the case in source.

Proposed resolution

In D7, the allowed vocabularies is a part of field settings which is not available in instance migration, which is where it needs to be in D8. We should add the settings first and then process it during migration.

For all the allowed vocabularies the vid needs to be added to the row so somewhere in the pipeline a migration_lookup can be done to determine the new, migrated vocabulary name.

Remaining tasks

Patch
Review
Testing - patch confirmed to work in #49
Commit

User interface changes

None

API changes

None

Data model changes

None