Clean up schema for table labels

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: created_at NOT NULL
• Missing constraint: updated_at NOT NULL
• Missing constraint: type NOT NULL
• Missing constraint: !(project_id IS NULL AND group_id IS NULL)
  • Since we don't really have a nice way of duplicating validations between the DB and models we should just implement this as a Rails validation
• Missing FK: project_id -> projects (35 entries violate this rule)
• Wrong data type: color should be int, save 6 MB
• Duplicate data: type could be enum/int, save 12 MB
• Index on (type, project_id) likely could be removed, type is not very selective
  • With the introduction of group labels this may no longer be the case