Work out why image folders are case-sensitive on Heroku but not locally

[frankieroberto]

If you use images referenced in the frontmatter, then the images need to be in a folder with the same name as the filename for the post (minus the date).

It seems this needs to be a case-sensitive match on Heroku, but not when running locally, which can cause much confusion. See https://github.com/nhsuk/manage-a-vaccination-campaign-design-history/pull/6

Probably it should also be case-sensitive locally so that people spot the error earlier? Or case-insensitive (eg maybe both post paths and image folders get downcased?) in all environments.