Don't include never-applied defined tags in tagUsageStatistics function
This removes the call to ChangeTags::listDefinedTags() in the function
tagUsageStatistics of ChangeTags. So only those in the change_tag table,
i.e. with a hitcount greater than zero, are returned.
Instead at SpecialTags, those tags in the list of defined tags, already
retrieved for other purposes, are appended with zero hitcount, when not
This incidentally makes it easier to get a list of tags applied at least
once, as needed for T27909 (where we don't want never applied tags).