Bug 1601094, rename the remaining .xul files in browser/ to .xhtml r=marionette-reviewers,whimboo,mossop

Differential Revision: https://phabricator.services.mozilla.com/D55751

--HG--
rename : browser/base/content/aboutDialog.xul => browser/base/content/aboutDialog.xhtml
rename : browser/base/content/newInstall.xul => browser/base/content/newInstall.xhtml
rename : browser/base/content/pageinfo/pageInfo.xul => browser/base/content/pageinfo/pageInfo.xhtml
rename : browser/base/content/safeMode.xul => browser/base/content/safeMode.xhtml
rename : browser/base/content/sanitize.xul => browser/base/content/sanitize.xhtml
rename : browser/base/content/titlebar-items.inc.xul => browser/base/content/titlebar-items.inc.xhtml
rename : browser/base/content/webext-panels.xul => browser/base/content/webext-panels.xhtml
rename : browser/base/content/webrtcIndicator.xul => browser/base/content/webrtcIndicator.xhtml
rename : browser/components/downloads/content/contentAreaDownloadsView.xul => browser/components/downloads/content/contentAreaDownloadsView.xhtml
rename : browser/components/migration/content/migration.xul => browser/components/migration/content/migration.xhtml
rename : browser/components/places/content/bookmarkProperties.xul => browser/components/places/content/bookmarkProperties.xhtml
rename : browser/components/places/content/bookmarksSidebar.xul => browser/components/places/content/bookmarksSidebar.xhtml
rename : browser/components/places/content/historySidebar.xul => browser/components/places/content/historySidebar.xhtml
rename : browser/components/places/content/places.xul => browser/components/places/content/places.xhtml
rename : browser/components/shell/content/setDesktopBackground.xul => browser/components/shell/content/setDesktopBackground.xhtml
extra : moz-landing-system : lando