Add sethelp/locks command

[chiizujin]

Brief overview of PR changes/additions

There was no way to change a help topic's locks in-game after it was created. This adds that functionality via a sethelp/locks command.

Motivation for adding to Evennia

New feature.

Examples

Tested as quelled superuser.

>sethelp/locks test
Current locks for entry 'test' are: read:all()

>sethelp/locks test = read:id(100)
Locks for entry 'test' changed to: read:id(100)

>help test
--------------------------------------------------------------------------------
No help found
(...)

>unquell
Account permissions (superuser) restored.

>sethelp/locks test
Current locks for entry 'test' are: read:id(100)

>sethelp/locks test=
Locks for entry 'test' reset to: read:all()

>sethelp/locks test
Current locks for entry 'test' are: read:all()

[Griatch]

This looks useful. Also interesting with the difference between locks.validate() and locks.add().