Adjusted header.html template to make menu responsive, collapsing on smaller screens

[adamadavidson]

Header.html template adjusted to make the navigation menu collapse behind a button on small screens. There should be no change when viewing these on tablets and larger screens.

Changes include:

• Making the navigation menu collapse on small screens, to be replaced by a "hamburger" icon that then expands to show the menu when clicked.
• Ensuring GitHub icon takes same colour/style as the navigation items
• Added the Bootstrap js script to the footer (in order to make the collapsing menu work). This code will have to be amedned to make the "local only" version work properly.

[aceberg]

Thanks for your contribution!