Jumbotron content update
Minor changes to page footers

Adds 'Edit This Page' to the homepage. Uses CSStables rather than
floats. No change to actual content.
For external dependencies, href to "https://..."

Was previously "//..." - handy for compatibility with both http and
https referring pages, but breaks file://... referring pages (e.g.
building a preview). Always use https, as all our Apache websites are
https based.
For external dependencies, href to "https://..."

Was previously "//..." - handy for compatibility with both http and
https referring pages, but breaks file://... referring pages (e.g.
building a preview). Always use https, as all our Apache websites are
https based.
Remove trailing slash from configured site.url

This is because in plugins/liquid templates, page.url includes a
*leading* slash
Define site structure in YAML front matter

Adds a site_structure plugin to parse this information, and modify base
template to use this when generating the navbar, instead of hardcoding.
navgroups now mostly automated (i.e. which tab is highlighted on each
page)
Add second-level menu support
This closes #3
This closes #4

Conflicts:
	_layouts/base.html -- forward slash is needed as part of path (not part of heading url)