add user menu

[kthull]

Adds the top-header user menu per the 2017 site style. Some considerations:

• this does not include the camp date/location because it lives in the even banner; depending on how the header is treated on internal pages, we may need to add this back
• last year's main nav had a log in link at mobile, where this year's does not, so the user menu remains at smaller breakpoints

[kthull]

We can also add Log in and Log out links to the main nav as the first link and hide first child at non-mobile breakpoints. Then on not-front, we can minimize or ditch the event banner and then hide the user menu in the top-header and instead show a date/location.