Add veterans.gc.ca menu to header

[smcmurtry]

Just the menu button, not the search box: https://app.zeplin.io/project/5acbd659dc553ec84805afd4/screen/5b758b92156c13b1cbd5a0a0

For this we can use the same approach as #1950, although we will also need to include some javascript from the VAC site to make the menu work.

[smcmurtry]

Implement a header and footer based on the new VAC header and footer

[smcmurtry]

Implement a header and footer based on the new VAC header and footer

[smcmurtry]

Here's where I got to with this:

• I used the approach described in VAC footer #1950 to create a component for the menu bar, removing the elements for the search box since this is not in the designs
• The menu button needs javascript to function, so as an initial attempt to get it working I copied all the imported js files from the html source for veterans.gc.ca into pages/_document.js
• The menu button then worked but the expansion was hidden behind other page content. I added a higher z-index value to the styling for the menu bar but this didn't fix the issue (although it's possible I didn't modify the index on the correct component).