Fix the summary arrow in recent Firefox
Fix the summary arrow in recent Firefox

This is a pretty hacky fix so I’ve tried to describe it as accurately as possible in the comments. It can be removed if either of the following happens:

* Mozilla fixes their summary arrow with display: inline-block
* Mozilla remove their support for @-moz-document

Fixes https://github.com/alphagov/govuk_elements/issues/221
gavboulton/govuk_elements,gavboulton/govuk_elements,gavboulton/govuk_elements,alphagov/govuk_elements,alphagov/govuk_elements,alphagov/govuk_elements,alphagov/govuk_elements,joelanman/govuk_elements,gavboulton/govuk_elements,joelanman/govuk_elements