[BUGFIX] Inserting custom element in RTE throws exception

Problem: Token validation fails.
Solution: Use BackendUtility::getModuleUrl to ensure validity of token.

Resolves: #58371
Releases: 6.2
Change-Id: I334826dbeb2a11a6b28d4fd610670fe4d7558e1a
Reviewed-on: https://review.typo3.org/30221
Tested-by: Chris Müller
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland