Make the X-UA-Compatible consistent across piwik codebase

plugins/Dashboard/templates/_header.twig

plugins/Login/templates/login.twig

plugins/Morpheus/templates/admin.twig

tests/resources/overlay-test-site/index.html

tests/resources/overlay-test-site/page-1.html

tests/resources/overlay-test-site/page-2.html

tests/resources/overlay-test-site/page-3.html

tests/resources/overlay-test-site/page-4.html

tests/resources/overlay-test-site/page-5.html

tests/resources/overlay-test-site/page-6.html