- Update the copyright year

- Remove copyright notices for code which is no longer vendored into the
  source tree but instead fetched from npm. In those cases the npm
  package includes machine-readable license metadata.

  I left the "annotator" notice because we still have some code derived from
  Annotator.js in the source tree, even though much of it has been
  replaced.

See also https://github.com/hypothesis/h/pull/5537