Improve sponsors tab layout (#3834)

Minimal improvement to sponsor logos on welcome screen.
Closes #3797

Note: IMHO ideally this layout would be rewritten to use flex display.
However in a spirit of keeping bugfixes targeted and minimal this implements
only necessary changes within exisiting block display.

src/css/main.less

@@ -1163,14 +1163,12 @@ dialog {
 }
 .tab_sponsor {
     display: none;
-    height: 50px;
-    max-height: 50px;
     margin: 0 auto 10px auto;
 }
 .img_sponsor {
     height: 100%;
-    padding-left: 10px;
-    padding-right: 10px;
+    max-height: 50px;
+    padding: 5px 10px;
 }
 .note {
     background-color: #fff7cd;