GlDrawer autodetect header height

Every time we use a drawer we have slightly different functions all very similar

Then some other places where we don't do that, but add margin-top with custom CSS.

Since drawer is always supposed to be below the header, we could default the top value to document.querySelector('header').getBoundingClientRect().bottom, and then (I think) never need to pass header-height or custom styles for it.

Or maybe .js-navbar or some safer selector.