Add separate function for right nav menu

Bug 1372536: Add a separate function to PluginArtefact, PluginInteraction and
PluginModule that lets you add menu entries especially to the right nav menu,
including top level entries.

The method right_nav in web.php still includes elements from the old menu_items
function for backwards compatibility but ignores all top level entries.

Change-Id: I91ba01f060a73575c483aa81aeb9201b11f1261f
Signed-off-by: Tobias Zeuch <tobias.zeuch@kit.edu>
Signed-off-by: Aaron Wells <aaronw@catalyst.net.nz>
Merge "Add separate function for right nav menu" into 1.10_STABLE