Clean up schema for table appearances

From 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: title NOT NULL (perhaps Unique?)
• Missing constraint: description NOT NULL
• Missing constraint: description_html NOT NULL
• Missing constraint: created_at NOT NULL
• Missing constraint: updated_at NOT NULL

We should also add a constraint (in the DB) that ensures only 1 row can be present.

assigned to @yorickpeterse

changed milestone to %9.5

mentioned in commit 90ceea25

mentioned in commit 26bb5041

closed via merge request !13433 (merged)

closed via commit 9c11894a

closed via commit 26bb5041

mentioned in commit 9c11894a

mentioned in commit bb4704b9

Possible MySQL build failure here: https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/29092856

mentioned in commit fea1e129

mentioned in commit 641ea063

mentioned in merge request !14785