Add pages_domains not null constraint for project_id

What does this MR do and why?

Add pages_domains not null constraint for project_id

Follow up for this MR: Add post migration step for removing null pages_domain

This is a non-optional association in the model, and there are no existing records on GitLab.com where this column is null (source).

Related: #442178 (closed)

Changelog: changed

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