Convert remaining rules to counters

[khiga8]

There are 5 rules which should be counters because counters have better disable functionality built in.

• GitHub::Accessibility::AvoidBothDisabledAndAriaDisabled
• GitHub::Accessibility::IframeHasTitle
• GitHub::Accessibility::ImageHasAlt
• GitHub::Accessibility::NoPositiveTabIndex
• GitHub::Accessibility::NoRedundantImageAlt