MergeRequestBlock validation isn't called

Description

It looks like none of the validation rules are being called for blocking merge requests.

The following discussion from !14934 (merged) should be addressed:

@pslaughter started a discussion:

issue: So it looks like this validation method isn't even called. @DouweM suspects it's because we use bulk_insert on these models. Let's handle this ~"bug" in a follow up issue.

Edited Aug 07, 2019 by Paul Slaughter