Disable menu when editing a message

#### Consistency

As stated here https://github.com/joomla/joomla-cms/pull/4694#issuecomment-59464421 when editing a message the main menu is still available. This differs from the rest of the admin area behavior. This PR unifies it.

#### test
Apply patch
go to /index.php?option=com_messages&view=message&layout=edit
Main menu should be disabled.