feat: Allow to use reduced scroll height when horizontal scroll - MEE…

…D-8026 - Meeds-io/MIPs#170 (#853)

Prior to this change, when using the `specific-scrollbar` class on an
horizontal scrollbar, the height still the same defined by the OS +
Browser. This change will use the same height of horizontal scrollbar as
it 's used in vertical scrollbar width.

platform-ui-skin/src/main/webapp/skin/less/mixins.less

@@ -28,6 +28,7 @@
     background-color: transparent;
     border-radius: 10px;
     width: 5px;
+    height: 5px;
   }
   &:hover {
     scrollbar-color: @greyColorLighten1;