Bitbucket URLs in `CHANGELOG.md` could be updated

[wilfwilson]

The old Bitbucket repo is now no longer available. Therefore, URLs such as https://bitbucket.org/james-d-mitchell/semigroups/issue/108 that exist in CHANGELOG.md are broken.

However, we (i.e. James) transferred the contents of these issues to the GitHub issue tracker when we made that migration. Therefore we could update the Bitbucket URLs to instead point at the corresponding GitHub URLs.

Unfortunately, this would require some manual work, since the numbering was not maintained. For example, https://bitbucket.org/james-d-mitchell/semigroups/issue/108 became #106 on GitHub (i.e. 108 → 106).

So it's probably best to give this job to a lowly individual such as an undergraduate or @mtorpey.