fix(welcome_page): Fix mobile version

* Fix layout
* Change background color & show image

css/_responsive.scss

 
 @media only screen and (max-width: $verySmallScreen) {
   .welcome {
+      display: block;
+
       #enter_room {
         position: relative;
         height: 42px;
       }
 
       .header {
-          background: #06345E;
-          background-image: linear-gradient(180deg, rgba(8, 110, 202, 0.8) 0%, rgba(8, 110, 202, 0) 100%);
+          background-color: #002637;
 
           #enter_room {
               .enter-room-input-container {
       .welcome-badge {
           margin-right: 16px;
       }
+
+      .welcome-footer {
+          display: none;
+      }
   }
 
     #videoResolutionLabel {

css/_variables.scss

 $welcomePageTitleColor: #fff;
 
 $welcomePageHeaderBackground: linear-gradient(0deg, rgba(0, 0, 0, 0.2), rgba(0, 0, 0, 0.2)), url('/images/welcome-background.png');
-$welcomePageHeaderBackgroundPosition: none;
+$welcomePageHeaderBackgroundPosition: center;
 $welcomePageHeaderBackgroundRepeat: none;
 $welcomePageHeaderBackgroundSize: cover;
 $welcomePageHeaderPaddingBottom: 0px;