optimize png images losslessly using zopflipng

app/assets/images/slider_handles.png

app/assets/images/touch-icon-ipad-retina.png

app/assets/images/touch-icon-ipad.png

app/assets/images/touch-icon-iphone-retina.png

app/assets/images/touch-icon-iphone.png

doc/administration/img/housekeeping_settings.png

doc/ci/build_artifacts/img/build_artifacts_browser.png

...ci/build_artifacts/img/build_artifacts_browser_button.png

doc/ci/img/builds_tab.png

doc/ci/img/features_settings.png

doc/ci/quick_start/img/build_log.png

doc/ci/quick_start/img/builds_status.png

doc/ci/quick_start/img/new_commit.png

doc/ci/quick_start/img/runners_activated.png

doc/ci/quick_start/img/single_commit_status_pending.png

doc/ci/quick_start/img/status_pending.png

doc/ci/runners/project_specific.png

doc/ci/runners/shared_runner.png

doc/ci/runners/shared_to_specific_admin.png

doc/ci/triggers/img/builds_page.png