Generic menu include required

[davecranwell]

We seem to have to write a menu for every site. It would be worth having some reusable code in this repo that can easily be plugged in.

Something suitable for:

• primary menu - the main areas/top sections of the site
• secondary menu - shows the current page's siblings and parent's siblings or similar.
• breadcrumbs - displays the current page's position within the site

Ideally we'd be able modify it with parameters. Such as how deep it goes, whether to show parents and their siblings, what level to consider "root" (in case of multiple sites/translations in one build)

[gasman]

Seems like this would be a good thing to have in Wagtail itself (either in core or contrib), since it's not just Torchbox projects (or projects using Torchbox's project standards) that would benefit from this. Best practice for creating menus is a question that's come up in e.g. https://github.com/torchbox/wagtail/issues/776 - so it would be nice to have a drop-in solution for projects created with wagtail start. (I suspect that pasting code in from wagtaildemo is the most common route right now...)