Change SiteTreeHints to check for canEdit

Allowing ability to have
public function canCreate($member = null) {
    return MyPage::get()->count() < 1;
}
and still be able to re-order this page in the sitetree.

Cleaned-up version of [this pull request][1].

[1]: https://github.com/silverstripe/silverstripe-cms/pull/1035