ExternalUserNames::applyPrefix() should probably apply the prefix to
otherwise-usable names that would fail User::isUsableName() only because
of a lowercase first letter (or other non-normalization that
User::getCanonicalName() fixes).
Bug: T211450
Change-Id: If695432d6b683442bd74ee160064c950b09c924a
This makes things centralized to reduce maintenance cost and also
enables me to use this methods in Wikibase to handle RC injection
Bug: T185034
Change-Id: Ic8c602e316144ccb5b05c69a0cc607cd53e38912