Adding new static header

[wilwade]

Problem

Matching the header to the design

Closes #82

Solution

• Static header added
• JS to manage header open/closed state
• Styles

with @claireolmstead

Screenshots (optional):

Hovers:

[shannonwells]

Reviewed the code changes and ran mdbook. There's about a 20px gap at 800px-819px where the mobile nav looks funny though, it's at an iPad width. I'm not sure if you want to fix it:

[wilwade]

There's about a 20px gap at 800px-819px where the mobile nav looks funny though, it's at an iPad width.

Added to the tail end fixes: #87