Some Project/Group imports are not importing all the relations due to
some *subrelations* (nested relations) being invalid. To avoid that, and
fix exceptions like
https://gitlab.com/gitlab-org/gitlab/-/issues/285107, every relation
created will, from now on, be validated.

Though, while working on
[enabling](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/48179)
this validation, some tests failed, exposing that some validations are
actually blocking some imports.

Validation to check if a _major_ association exists, or follow some kind
of requirements, like "DiffNote validating if the MergeRequest is
associated", doesn't work in the import process because the
relations are created in a _nested_ way. Something like:

```
diff_note = DiffNote.create(diff_note_attributes)
mr = MergeRequest.create(mr_attributes, diff_notes: [diff_note])
```

This commit skip these validations on the importing process, using the
`validates :validation, unless: :importing?` pattern.

Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/293705