Add navigation to school management forms

[ldodds]

Initial work to implement new index page and nav bar for school management forms.

• Adds a new school settings page to provide an index to the management forms and other options
• Adds a new component for the side bar to be added to this page and other school forms. Using a partial to make it easier to test independently.
• Adds a new partial to make it easier to add the new header and navigation to existing pages

Initially I've only added the navigation to the admin forms so we can review and merge this before changing the other forms. The others will be relatively easy to switch to use this layout although the tariffs, meters and possibly calendar pages are quite large.

• add body of settings page, listing key sections from nav, prompts to finish configuration, admin options and reports
• Add specs for navigation
• Only link to expert analysis if school has gas
• Add navigation to admin forms