[Tails-ux] #7257: Write the code to have submenus on the sid…

I finally review the prototype of submenus in the sidebar that esperal
added to the Redmine a while ago. And it's great!

So I pushed it to a topic branch:

    web/7257-sidebar-submenus

I think that the code needs to be reviewed quite a bit, but it's great
to have a first prototype and something to play with!

And it's all HTML + CSS, so no hack needed to have this working in
ikiwiki. Congrats!

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sajolida