Reduce db calls on normal invite-based signup

This removes an unnecessary database call after creating a user, and documents `db.CreateUser()` to make it clear that extra calls are unnecessary.

account.go

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2018-2019 A Bunch Tell LLC.
+ * Copyright © 2018-2020 A Bunch Tell LLC.
  *
  * This file is part of WriteFreely.
  *
@@ -168,11 +168,7 @@ func signupWithRegistration(app *App, signup userRegistration, w http.ResponseWr
 
 	// Log invite if needed
 	if signup.InviteCode != "" {
-		cu, err := app.db.GetUserForAuth(signup.Alias)
-		if err != nil {
-			return nil, err
-		}
-		err = app.db.CreateInvitedUser(signup.InviteCode, cu.ID)
+		err = app.db.CreateInvitedUser(signup.InviteCode, u.ID)
 		if err != nil {
 			return nil, err
 		}

database.go

@@ -178,6 +178,7 @@ func (db *datastore) dateSub(l int, unit string) string {
 	return fmt.Sprintf("DATE_SUB(NOW(), INTERVAL %d %s)", l, unit)
 }
 
+// CreateUser creates a new user in the database from the given User, UPDATING it in the process with the user's ID.
 func (db *datastore) CreateUser(cfg *config.Config, u *User, collectionTitle string) error {
 	if db.PostIDExists(u.Username) {
 		return impart.HTTPError{http.StatusConflict, "Invalid collection name."}