Fill in remaining missing pieces

- Database schema changes, removing obsolete custom domain-related code
- Missing user structs
- Setup verbiage changes
- Missing routes
- Missing error messages

auth.go

@@ -0,0 +1,18 @@
+package writefreely
+
+// AuthenticateUser ensures a user with the given accessToken is valid. Call
+// it before any operations that require authentication or optionally associate
+// data with a user account.
+// Returns an error if the given accessToken is invalid. Otherwise the
+// associated user ID is returned.
+func AuthenticateUser(db writestore, accessToken string) (int64, error) {
+	if accessToken == "" {
+		return 0, ErrNoAccessToken
+	}
+	userID := db.GetUserID(accessToken)
+	if userID == -1 {
+		return 0, ErrBadAccessToken
+	}
+
+	return userID, nil
+}