Fixed permission checks when managing/deleting a thread

[Almighty-Satan]

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Changes

• [X] Internal code
• [ ] Library interface (affecting end-user code)
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Closes Issue: #2188

Description

When calling ThreadChannel#getManager or ThreadChannel#delete it is currently checked if the user has the MANAGE_CHANNEL permission, but the permission actually required to manage or delete a thread is MANAGE_THREADS.