Merge branch '8.x-1.x' into 2.x

.gitlab-ci.yml

composer.json

config/schema/diff.schema.yml

diff.info.yml

src/Form/FieldsSettingsForm.php

src/Form/RevisionOverviewForm.php

src/Plugin/diff/Field/CoreFieldBuilder.php

...s/diff_test/config/optional/views.view.test_diff_view.yml

tests/src/Functional/CoreVersionUiTestTrait.php

tests/src/Functional/DiffRevisionTest.php