gollum: 4.1.4 -> 5.0.1

Some changes might require manual migration steps:
"Due to changes to the way in which Gollum handles filenames, you may
have to change some links in your wiki when migrating from gollum 4.x.
See the release notes [0] for more details. You may find the
bin/gollum-migrate-tags script helpful to accomplish this. Also see the
--lenient-tag-lookup option for making tag lookup backwards compatible
with 4.x, though note that this will decrease performance on large wikis
with many tags." (source: [1])

[0]: https://github.com/gollum/gollum/wiki/5.0-release-notes
[1]: https://github.com/gollum/gollum/blob/v5.0.0/HISTORY.md