Navigation and CMS Rebuild

Current Navigation structure builder is rubbish - make it better.

Navigation

• Migrate Journal nav booleans to Nav elements
• Remove all nav booleans from Journal object
• Update navigation generation to make it more understandable/useable
• Allow user to make a link external or select an existing CMS page
• All nav entries should use the nav_element html file so that they are all the same. Nav Elements are at `theme/name/elements/nav_element.html
• Navigation pages should obey element ordering

CMS

All pages should be CMS pages to that end we want to make each of our "fixed" pages into elements and allow you to add multiple elements to a page so you could have

• Issues Page with an HTML block followed by an Issue List block
  or
• Contact Page with a Contact Block followed by an HTML block

and so on. The should also work for the homepage and allow blocks to be side by side in columns.