Update Testimonials Code To Use Hakyll Public API

[rebeccaskinner]

Note: Depends on #108

We should update the implementation of the testimonials code to no longer use hakyll internal modules.

[philderbeast]

@rebeccaskinner is this still an issue? If so, what would the fix involve?

[rebeccaskinner]

@philderbeast I don't think it's an issue anymore. I'm not sure when we fixed this but I suspect a while ago and we just forgot to link this issue. It looks like your PR #306 also addresses the updates, so I'm going to close this issue and also approve that PR.