Allow hosting files from a custom directory

[GentlemanHal]

This would allow users to select local images (dialog or DnD) for use on the success screen.

This would be possible by storing the encoded image in local storage. We would need to test this to see if we get issues trying to store large blobs of data locally.

Edit: I'm thinking this might be better as an option on the server which allows it to serve up a local directory. This would be disabled on nevergreen.io for obvious reasons but would work for people self hosting, which appears to be most people.

[GentlemanHal]

I have decided I'd rather this be a server side option.

To make sure this isn't enabled by accident, this should be off by default. To turn it on the user can pass a new environment variable CUSTOM_HOSTING=enabled or something (exact name and value TBC). If this variable is given with the correct value then a second environment variable must be passed with the directory to host from e.g. CUSTOM_HOSTING_PATH=/whatever (name also TBC)

We can then hopefully just pass this path to wrap-defaults in app_routes.clj to get Ring to automatically serve up any files in the directory 🎉

We would have to make sure this path doesn't override the default public directory we serve the client from.

I'm also not entirely sure how we'd test this, as we'd be relying on Ring to do the heavy lifting. This might have to be a new integration test that actually starts up the server and tries to GET a custom hosted file 🤔 Note: we don't currently have any integration tests that actually start the server.