Revert "fix(welcome-page): remove watermark container to avoid z-index wars"

This reverts commit 890151fa72.

css/_welcome_page.scss

             font-size: 32px;
         }
     }
+
+    .welcome-watermark {
+        position: absolute;
+        width: 100%;
+        height: 100%;
+    }
 }

react/features/welcome/components/WelcomePage.web.js

                 className = { `welcome ${showAdditionalContent
                     ? 'with-content' : 'without-content'}` }
                 id = 'welcome_page'>
-                <Watermarks />
+                <div className = 'welcome-watermark'>
+                    <Watermarks />
+                </div>
                 <div className = 'header'>
                     <div className = 'welcome-page-settings'>
                         <SettingsButton