Fix phan errors in ChangesListSpecialPage (#6)

Fix phan errors in ChangesListSpecialPage (#6)

Specifically in registerFiltersFromDefinitions(), phan was unable to
tell that $groupDefinition['class'] was a string. I think it assumed it
was an integer because the earlier $groupDefinition['priority'] was an
integer.

Use phan's more advanced type documentation system to instruct it that
$className will be a string.

Change-Id: I09f41366b713d6159b57e798cfd1720f42ba30ef