Git has two types of tags, lightweight tags and annotated tags. Only
annotated tags have a "tag date" associated with them. Recently it
appears that releases have been tagged with lightweight tags rather than
annotated tags, which the `generate-change-list.js` script did not
handle.

 - When fetching the last tag, use the commit date as a fallback for the
   tag date, and bail with an error if no date can be found

 - Link to the HTML rather than JSON representation of a PR in release
   notes

 - Fix a warning about use of deprecated GitHub API pagination methods
   by switching to the pagination APIs described at
   https://octokit.github.io/rest.js/#pagination

 - Support running the `generate-change-list.js` script as a CLI tool
   for debugging using `node generate-change-list.js <tag name>`

 - Skip Dependabot PRs in the change list, as these add a lot of noise.
   Generally readers are going to be interested in non-maintenance PRs.