Make GlDrawer body scrollable
GlDrawer component currently doesn't support scrolling when contents do not bit vertically. Only workaround is to handle the scrolling on app itself where the component is being used. While that's sufficient for now, but in the long term where we want to use this drawer in multiple places, having a native scrolling support is ideal.
Reason why we need to do this at component-level is because in current Issuable right sidebar, contents are scrollable but we mask the scrollbar by setting container width smaller than items width, which leads independent scroll regions between page and sidebar;