Add $wgSemiprotectedRestrictionLevels

It's possible that a wiki could introduce new protection levels that
should be considered "semiprotected". For example, if an
"emailconfirmed" protection level were added and an appropriate entry
were made in $wgAutopromote, that might be considered semi-protection
since anyone can automatically gain the ability to edit those pages
merely by setting and confirming their email address.

The most straightforward way to take care of this is to add a config
variable to specify which protection levels are considered
"semiprotected". So let's do that.

Also, let's take the opportunity to make
$title->isSemiProtected( 'create' ) works correctly.

Bug: 43462
Change-Id: Ic9db6ff6cbd84bd9734be09efbea5a5891197fa0