Clean up schema for table releases

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Per https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):

• Wrong data type: created_at should be timestamptz
• Wrong data type: updated_at should be timestamptz
• Missing constraint: created_at NOT NULL
• Missing constraint: updated_at NOT NULL
• Missing constraint: project_id NOT NULL
• Missing FK: project_id -> projects (32 entries violate this rule)
• Duplicate data: description and description_html - not possible to have one format which can generate both..?
• Duplicate index: no need for an index on project_id when there is one on project_id, tag
• Missing constraint: UNIQUE (project_id, tag) (2 entries violate this rule)