Make navbar static everywhere

Also remove unused page layout and use the same HTML strucure above all
pages.
Add 'wdm' gem to Gemfile (required for Windows to '--watch' Jekyll)
Add TODO
Update navbar, footer, index, prices and contact

Navbar now use the logo and active page the brand color.
Footer stacks instead of inline.
Index has now a partner-logo section.
Prices use a first try of header.
Contact use address element.