feat(Watermarks): Add deafault logo prop.

css/_variables.scss

 $watermarkWidth: 186px;
 $watermarkHeight: 74px;
 
+$welcomePageWatermarkWidth: 186px;
+$welcomePageWatermarkHeight: 74px;
+
 /**
  * Welcome page variables.
  */

css/_welcome_page.scss

         position: absolute;
         width: 100%;
         height: 100%;
+
+        .watermark.leftwatermark {
+            width: $welcomePageWatermarkWidth;
+            height: $welcomePageWatermarkHeight;
+        }
     }
 }

interface_config.js

     DEFAULT_LOCAL_DISPLAY_NAME: 'me',
     DEFAULT_LOGO_URL: 'images/watermark.png',
     DEFAULT_REMOTE_DISPLAY_NAME: 'Fellow Jitster',
+    DEFAULT_WELCOME_PAGE_LOGO_URL: 'images/watermark.png',
 
     DISABLE_DOMINANT_SPEAKER_INDICATOR: false,
 

react/features/base/react/components/web/Watermarks.js

      */
     _welcomePageIsVisible: boolean,
 
+    /**
+     * The default value for the Jitsi logo URL.
+     */
+    defaultJitsiLogoURL: ?string,
+
     /**
      * Invoked to obtain translated strings.
      */
         let reactElement = null;
         const {
             _customLogoUrl,
-            _customLogoLink
+            _customLogoLink,
+            defaultJitsiLogoURL
         } = this.props;
 
         if (this._canDisplayJitsiWatermark()) {
             const link = _customLogoLink || this.state.jitsiWatermarkLink;
             const style = {
-                backgroundImage: `url(${_customLogoUrl || interfaceConfig.DEFAULT_LOGO_URL})`,
+                backgroundImage: `url(${_customLogoUrl || defaultJitsiLogoURL || interfaceConfig.DEFAULT_LOGO_URL})`,
                 maxWidth: 140,
                 maxHeight: 70
             };

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

      */
     render() {
         const { _moderatedRoomServiceUrl, t } = this.props;
-        const { APP_NAME } = interfaceConfig;
+        const { APP_NAME, DEFAULT_WELCOME_PAGE_LOGO_URL } = interfaceConfig;
         const showAdditionalContent = this._shouldShowAdditionalContent();
         const showAdditionalToolbarContent = this._shouldShowAdditionalToolbarContent();
         const showResponsiveText = this._shouldShowResponsiveText();
                     ? 'with-content' : 'without-content'}` }
                 id = 'welcome_page'>
                 <div className = 'welcome-watermark'>
-                    <Watermarks />
+                    <Watermarks defaultJitsiLogoURL = { DEFAULT_WELCOME_PAGE_LOGO_URL } />
                 </div>
                 <div className = 'header'>
                     <div className = 'welcome-page-settings'>