fix(welcome_page): Update header to latest design & use generic key name

css/_variables.scss

 $welcomePageHeaderTextAlign: center;
 
 $welcomePageHeaderContainerDisplay: flex;
-$welcomePageHeaderContainerMargin: 146px 32px 0 32px;
+$welcomePageHeaderContainerMargin: 104px 32px 0 32px;
 
 $welcomePageHeaderTextTitleMarginBottom: 0;
 $welcomePageHeaderTextTitleFontSize: 42px;

css/_welcome_page.scss

         background-repeat: $welcomePageHeaderBackgroundRepeat;
         background-size: $welcomePageHeaderBackgroundSize;
         padding-bottom: $welcomePageHeaderPaddingBottom;
-        background-color: #002637;
-        height: 480px;
+        background-color: #131519;
+        height: 400px;
         overflow: hidden;
         position: relative;
 

lang/main.json

         "getHelp": "Get help",
         "go": "GO",
         "goSmall": "GO",
+        "headerTitle": "Jitsi Meet",
+        "headerSubtitle": "Secure and high quality meetings",
         "info": "Dial-in info",
         "join": "CREATE / JOIN",
-        "jitsiMeet": "Jitsi Meet",
         "jitsiOnMobile": "Jitsi on mobile – download our apps and start a meeting from anywhere",
         "moderatedMessage": "Or <a href=\"{{url}}\" rel=\"noopener noreferrer\" target=\"_blank\">book a meeting URL</a> in advance where you are the only moderator.",
         "privacy": "Privacy",
         "roomname": "Enter room name",
         "roomnameHint": "Enter the name or URL of the room you want to join. You may make a name up, just let the people you are meeting know it so that they enter the same name.",
         "sendFeedback": "Send feedback",
-        "secureMeetings": "Secure and high quality meetings",
         "startMeeting": "Start meeting",
         "terms": "Terms",
         "title": "Secure, fully featured, and completely free video conferencing"

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

                     <div className = 'header-image' />
                     <div className = 'header-container'>
                         <h1 className = 'header-text-title'>
-                            { t('welcomepage.jitsiMeet') }
+                            { t('welcomepage.headerTitle') }
                         </h1>
                         <span className = 'header-text-subtitle'>
-                            { t('welcomepage.secureMeetings')}
+                            { t('welcomepage.headerSubtitle')}
                         </span>
                         <div id = 'enter_room'>
                             <div className = 'enter-room-input-container'>