Use fetchLock() instead of isLockedBy() to avoid 403 errors

Problem/Motivation

If "Content" is not enabled in the Content Lock config form, it returns a 403 error when trying to add a block

Steps to reproduce

- Install the module
- Disable "Content" from the Content Lock module configuration form
- Try to edit layout builder inside a node (i.e. landing page)
- See 403 error in browser console

Proposed resolution

Use fetchLock() instead of isLockedBy() from ContentLock.php to verify that a lock exists and that the user is the owner of the lock