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/comment_domain_get.go

@@ -0,0 +1,24 @@
+package main
+
+import ()
+
+func commentDomainGet(commentHex string) (string, error) {
+	if commentHex == "" {
+		return "", errorMissingField
+	}
+
+	statement := `
+    SELECT domain
+		FROM comments
+		WHERE commentHex = $1;
+	`
+	row := db.QueryRow(statement, commentHex)
+
+	var domain string
+	var err error
+	if err = row.Scan(&domain); err != nil {
+		return "", errorNoSuchDomain
+	}
+
+	return domain, nil
+}