Make Annotation and Notes tab bar stick to top of the viewport when the
annotation list is scrolled down.

This relies on support for `position: sticky` which is available in
recent versions of Firefox and Safari*. On browsers where
`position: sticky` is not supported, the tab bar will just scroll
off-screen when scrolling the annotation list down.

Because annotation cards may now be displayed under the tabs, add a
background color, Z index and negative horizontal margin to the tabs
component so that any part of an annotation card under the tab bar is
not visible.

* In Chrome v52 this feature is available but only when the
  'Experimental web platform features' flag is on.