Control user access to models in Keystone.js

## index.js
// Place this with the other middleware inclusion in routes/index.js

keystone.pre('admin', middleware.enforcePermissions);

## middleware.js
// Place this in routes/middleware.js

/**
  Sets navigation and enforces permissions specified in the user models
 */
exports.enforcePermissions = function (req, res, next) {
	var nav = {
		blog: ['blog', 'tag'],
		about: ['page', 'category'],
		access: 'users',
	};
	keystone.set('nav', nav);
	if (req.user) {
		// This assumes users have a set of boolean fields, "permBlog", "permAbout", etc.
		// which control access to these sets of navigation items.
		var hideLists = (name, hidden) => keystone.list(name).set('hidden', hidden);
		['Blog', 'Tag'].map(list => hideLists(list, !req.user.permBlog));
		['Page', 'Category'].map(list => hideLists(list, !req.user.permAbout));
		['User'].map(list => hideLists(list, !req.user.permAdmin));
		!req.user.permBlog && delete nav.blog;
		!req.user.permAbout && delete nav.about;
		!req.user.permAccess && delete nav.access;
		keystone.nav = keystone.initNav(nav);
	}
	next();
}