Fixes issues with show whitespace button in diffs

Closes #42597 and #58852

See merge request gitlab-org/gitlab-ce!28242

Decouples sidebar collapsed style from
behavior.

Correctly updates the URL without overwriting parameters
Reloads the diff file content without reloading the page

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58852, https://gitlab.com/gitlab-org/gitlab-ce/issues/42597

Simplify frontend test fixtures

See merge request gitlab-org/gitlab-ce!27531

for f in $(find spec/javascripts/fixtures/ -name '*.rb')
do
  sed -E -e 's/(, ){0,1}(fixture_file_name|
  example\.description)(, ){0,1}//' < $f > $f.tmp; mv $f.tmp $f
done

for f in $(find spec/javascripts/fixtures/ -name '*.rb')
do
  sed 's/do |example|/do/' < $f > $f.tmp; mv $f.tmp $f
done

for f in $(find spec/javascripts/fixtures/ -name '*.rb')
do
  grep -v store_frontend_fixture < $f > $f.tmp; mv $f.tmp $f
done