GTNPORTAL-3370 - Small adjustments to the "keep page private" option.

On IRC, it was discussed how this feature should behave. So, a few adjustments were made:

- When an user tries to access a node and the access is restricted, then the "page not found" behavior is presented. In other words: the default page is shown, without warnings to the user.
- For new pages, the checkbox to restrict access to the page outside of the publishing window is enabled by default.
- Changed the label, to make it clearer that this is about restricting access to the page, not just hidding the page.