7. Have a twig function to link speakers

[dbu]

This pull request adds a twig extension providing the speaker_path twig function to link to a speaker. This is a clean way to inject the additional information we need to build the path, and keep the logic encapsulated. If we later decide we need a different logic for the anchors, we only need to update in one place.

No data changes, so no need to reload the fixtures this time.