Canvis a "Ability to remove/hide menu items"

@verarojman asked on gitter whether it was possible to somehow remove menu items through the decidim.rb initializer. I investigated a bit and it looks like the menu subsystem is append-only at this point.

It is possible to hide them using CSS/JS hacks, but that's obviously brittle and can have unexpected consequences.

Being able to edit the menu through the initializer (or through the admin dashboard) seems like a sensible expectation from Decidim instance maintainers, to me. And from what I could tell, it's viable to extend the menu subsystem to allow for this within its current architecture.

Not sure how high a priority this would be, of course. Nor how common this demand is.

What do you all think?

(I did a quick search for a similar issue here on meta and on github, couldn't find one - let me know if this is a duplicate)