Clean up schema for table chat_teams

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

• Empty table
• Wrong data type: created_at should be timestamptz
• Wrong data type: updated_at should be timestamptz
• Missing contraint: team_id NOT NULL
• Missing contraint: name NOT NULL (if necessary)
• Duplicate data: team_id implies name, might not be necessary?
• Missing foreign key: team_id (with a cascading delete)