In preparation for executing the client release process entirely on
Jenkins, make update-changelog.js automatically update CHANGELOG.md with
a list of changes since the most recent tag, instead of requiring a
developer to do this manually.

This will lose the advantages of having a changelog written by humans, but
we primarily use other channels nowadays to notify staff and users about
Hypothesis product changes.

 - Add scripts/generate-change-list.js which generates a nicely
   formatted list of changes since the last release.
 - Use functions from generate-change-list.js to auto-populate section
   for new release in CHANGELOG.md when running `yarn version`.