Make about:logo work in Thunderbird b=365963 r=mscott

mail/base/content/aboutDialog.css

@@ -4,7 +4,7 @@
 
 #clientBox {
   background-color: #FFFFFF;
-  background-image: url("chrome://branding/content/about-thunderbird.png");
+  background-image: url("about:logo");
   background-repeat: no-repeat;
   padding-top: 216px;
   color: #000000;

mail/base/jar.mn

@@ -53,7 +53,7 @@ messenger.jar:
 #ifdef MOZ_USE_GENERIC_BRANDING
 % content branding %content/branding/ xpcnativewrappers=yes
     content/branding/about-credits.png              (content/about-credits.png)
-    content/branding/about-thunderbird.png          (content/about-thunderbird.png)
+    content/branding/about.png                      (content/about-thunderbird.png)
     content/branding/thunderbird-watermark.png      (content/thunderbird-watermark.png)
     content/branding/icon64.png                     (content/icon64.png)
 #ifdef XP_WIN

other-licenses/branding/thunderbird/content/jar.mn

@@ -1,7 +1,7 @@
 messenger.jar:
 % content branding %content/branding/ xpcnativewrappers=yes
   content/branding/about-credits.png                (about-credits.png)
-  content/branding/about-thunderbird.png            (about-thunderbird.png)
+  content/branding/about.png                        (about-thunderbird.png)
   content/branding/thunderbird-wizard-badge.png     (thunderbird-wizard-badge.png)
   content/branding/thunderbird-watermark.png        (thunderbird-watermark.png)
   content/branding/icon64.png                       (icon64.png)