Clean up header styles in preparation for responsive nav

* Fix indentation in LESS files
* Make full-width (``.container_12``) containers auto-width on
  narrow displays.
* Use IDs rather than classes for singular elements

Addresses #774
Move "My Account" menu into main megamenu

I think this will make responsiveness easier.  Also, I've always
hated how it kind of hung out there on the right.
Update changelog for 0.18.0
Merge branch 'develop' into 774-responsive_design