Improve default permissions of Drafts page

[hissy]

Draft pages inherits their permissions from their parent page (usually it is Home page). When we want to create a limited user account that will be allowed to create some types of page, we have to change the permissions of Draft (/!drafts) page. If not, the limited user will get weird "Not Found" page when creating a new draft page, because the user can't access to editing permissions of "Home" page. It is not intuitive. This change helps people who wants to setup Advanced Permission mode.

1. /!drafts page should have own permissions, not inherit from Home page
2. Assign "Page Owner" to some permissions that are required to access to Composer interface
3. A new draft page inherits its permissions from /!drafts page.

[aembler]

Nice catch

[hissy]

Thanks!