Fix Title::loadRestrictions() for create-protected titles

Fix Title::loadRestrictions() for create-protected titles

Title::loadRestrictions() returns restriction levels, not the
corresponding user rights. These are mostly identical except for the
backwards-compatibility levels "sysop" and "autoconfirmed".

For create protection it calls Title::getTitleProtection() to fetch
them, but Ic5026384 changed that to return rights. Split that function
into two, an internal method that returns restriction levels for
loadRestrictions() to call and the public method that converts levels
into rights.

Bug: T85108
Change-Id: I3ea4cb9c5e37b746402744dd7a883763ee07195a