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/commenter.go

@@ -0,0 +1,38 @@
+package main
+
+import (
+	"time"
+)
+
+type commenter struct {
+	CommenterHex string    `json:"commenterHex,omitempty"`
+	Email        string    `json:"email,omitempty"`
+	Name         string    `json:"name"`
+	Link         string    `json:"link"`
+	Photo        string    `json:"photo"`
+	Provider     string    `json:"provider,omitempty"`
+	JoinDate     time.Time `json:"joinDate,omitempty"`
+}
+
+func commenterIsProviderUser(provider string, email string) (bool, error) {
+	if provider == "" || email == "" {
+		return false, errorMissingField
+	}
+
+	statement := `
+    SELECT EXISTS (
+      SELECT 1
+      FROM commenters
+      WHERE email=$1 AND provider=$2
+    );
+  `
+	row := db.QueryRow(statement, email, provider)
+
+	var exists bool
+	if err := row.Scan(&exists); err != nil {
+		logger.Errorf("error checking if provider user exists: %v", err)
+		return false, errorInternal
+	}
+
+	return exists, nil
+}