Add before_validation :update_project_id

Kerri Millerrequested to merge
512949-add-before_validation-validation-of-project_id into master

What does this MR do and why?

We need to ensure that project_id is present, so we populate in this before_validation.

This is similar to the work proposed for merge_request_diff_commits.

Related to #512949 (closed) and #534151 (closed)

Edited by Kerri Miller

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