Exclude users without view_page permission for page-specific permissions

[timobrembeck]

Describe the Bug

At the moment, users, who don't have the permission to view the page tree at all can be granted the permission to edit specific pages. This does not make sense, since the users are not allowed to open the page form if they are not allowed to view pages.

Steps to Reproduce

1. Enable page-specific permissions
2. See e.g. event manager role in selection field

Expected Behavior

Users without the permission to view pages should be excluded from the possible choices

Actual Behavior

They are shown

Additional Information

Together with #1494, this hopefully makes the feature usable.
Maybe we should also enable page-specific permissions in the test data as well, so we notice potential problems earlier.