Add max-width for context menu

Auditors: @bsclifton

Fix #4272

This is just a quick fix to avoid the situation of bookmarks overflow
menu being too big and unusable.

I should note this doesn't fully fix other issues with that menu
including submenus when width isn't big enough and if there is a
subfolder after you have to scroll down.

For those things I think we should investigate doing a popup window for
sub-menus.

less/contextMenu.less

@@ -58,6 +58,7 @@
 }
 
 .contextMenuItem {
+  max-width: 420px;
   padding: 6px 10px 6px 20px;
   &.hasIcon {
     padding-left: 10px;