Revert "removed semibold font-face"

This reverts commit 059028da02.

src/UI/Content/font.less

@@ -22,6 +22,18 @@
          url('./fonts/opensans-regular.ttf') format('truetype')
 }
 
+@font-face {
+    font-family: 'Open Sans';
+    font-style: normal;
+    font-weight: 600;
+    src: url('./fonts/opensans-semibold.eot');
+    src: local('Open Sans SemiBold'),
+         local('OpenSans-SemiBold'),
+         url('./fonts/opensans-semibold.eot?#iefix') format('embedded-opentype'),
+         url('./fonts/opensans-semibold.woff') format('woff'),
+         url('./fonts/opensans-semibold.ttf') format('truetype')
+}
+
 @font-face {
     font-family: 'Ubuntu Mono';
     font-style: normal;