[TwigBundle] Use display instead of visibility for exception page icons
merged branch igorw/css-display (PR #6624)

This PR was merged into the master branch.

Discussion
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[2.3][TwigBundle] Use display instead of visibility for exception page icons

Originally spawned from #6612, this allows us to get rid of the ugly margins for the open and close icons on the exception page.

Commits
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e66bd14 [TwigBundle] Use display instead of visibility for exception page icons