api: Add go files

I know this is a huge commit, but I can't be bothered to check
this in part by part.

api/domain_ownership_verify.go

@@ -0,0 +1,26 @@
+package main
+
+import ()
+
+func domainOwnershipVerify(ownerHex string, domain string) (bool, error) {
+	if ownerHex == "" || domain == "" {
+		return false, errorMissingField
+	}
+
+	statement := `
+		SELECT EXISTS (
+			SELECT 1
+			FROM domains
+			WHERE ownerHex=$1 AND domain=$2
+		);
+	`
+	row := db.QueryRow(statement, ownerHex, domain)
+
+	var exists bool
+	if err := row.Scan(&exists); err != nil {
+		logger.Errorf("cannot query if domain owner: %v", err)
+		return false, errorInternal
+	}
+
+	return exists, nil
+}