Issue #2550945 by alexpott, joelpittet, xjm, Wim Leers, stefan.r: Add Html::escape()

core/lib/Drupal/Component/Utility/Html.php

core/lib/Drupal/Component/Utility/SafeMarkup.php

core/lib/Drupal/Core/Render/Element/HtmlTag.php

core/lib/Drupal/Core/Template/Attribute.php

core/lib/Drupal/Core/Template/AttributeArray.php

core/lib/Drupal/Core/Template/AttributeBoolean.php

core/lib/Drupal/Core/Template/AttributeString.php

core/lib/Drupal/Core/Template/AttributeValueBase.php

core/modules/aggregator/src/Tests/AggregatorTestBase.php

core/modules/system/src/Tests/Menu/LocalActionTest.php

core/modules/system/src/Tests/Menu/LocalTasksTest.php

core/tests/Drupal/Tests/Component/Utility/HtmlTest.php

core/themes/engines/twig/twig.engine