Migrate Drupal 7 comment entity translations data to Drupal 8

Unpostponed as #2980996: Migrate Drupal 7 taxonomy term entity translations data to Drupal 8 and #2981392: Comment migration corrupts data with multilingual sites landed.

I'll do a proper review and manual test tomorrow morning my time, but spotted one nit at the first glance.

+++ b/core/modules/comment/src/Plugin/migrate/source/d7/CommentEntityTranslation.php
@@ -0,0 +1,103 @@
+/**
+ * Drupal 7 comment entity translation source from database.
+ */
+class CommentEntityTranslation extends FieldableEntity {

My default nit on API doc standards. Third person verb, please :)

Markus

And some more.

1. 
b/core/modules/comment/tests/src/Kernel/Migrate/d7/MigrateCommentTest.php
@@ -40,6 +42,7 @@ protected function setUp() {
+    $this->installEntitySchema('taxonomy_term')

Is this needed?

2. Typo

b/core/modules/content_translation/migrations/d7_comment_entity_translation.yml
new file mode 100644
index 0000000..9dea06c
--- /dev/null
+++ b/core/modules/content_translation/migrations/d7_comment_entity_translation.yml
@@ -0,0 +1,28 @@
+migration_dependencies:
+  kequired:

I spent the morning on testing and reviewing patch #12. My manual tests were quite extensive with inspections in both UI and database. My test site had English and Finnish enabled.

A. On one content type the comments had Entity Translation enabled together with Title, with additional language neutral and translatable fields.

• I tested standalone comments in language neutral, Finnish, English. The language was correctly migrated in all of them.
• I tested the actual Entity Translations of comments, including the 'published' and 'flag translations outdated' metadata. The translations and their metadata was correctly migrated.
• Conclusion: test passed

B. I also manually tested that we are not causing regression (not that I would not trust our test coverage, but wanted to check this manually as well)

• On another content type I did not have the Entity Translation / Title enabled.
• I wanted to see that the comments and their languages were correctly migrated.
• Results were as expected.
• Conclusion: my manual regression test passed.

About the patch itself:

• My earlier feedback in #9 and #10 has been addressed.
• I read the whole patch again and could not find anything to complain.
• The test coverage looks good to me.

Conclusion of the review and test: RTBC.

Great work @maxocub! These Entity Translation migrations are conceptually among the most complex ones we have but you mastered them perfectly. Woop, woop!

Cheers,
Markus

Actually, let's run the testbot still for PostgreSQL and SQLite just to be sure.

Tests are green also on PostgreSQL and SQLite. Back to RTBC.

Committed and pushed e0ab51cf25 to 8.7.x and 4e430e94e5 to 8.6.x. Thanks!

Discovered a bug in this: #3178966: Fatal error in CommentEntityTranslation @MigrationSource when source site does not have comment or node module installed.