treewide: fix linting errors

src/policies.json

src/userChrome.css

src/userChrome/about3Pane.css

src/userChrome/accountCentral.css

src/userChrome/messagePane.css

src/userChrome/messenger.css

src/userChrome/messengerCompose.css

src/userChrome/threadCard.css

src/userChrome/threadPane.css

src/userChrome/unifiedToolbar.css

src/userContent.css

src/userContent/aboutAccountSetup.css

src/userContent/aboutImport.css

src/userContent/aboutSettings.css

src/userContent/messagePane.css