Replace string format by os.path.join wherever we want to create file paths

[BeritJanssen]

This surfaced in #950: the RELOAD_PARTICIPANT_URL had a leading slash in the production settings, leading to problems with social media sharing. We should go over all the places where format strings are used to create file paths or urls. For file paths, we can use os.path.join instead, which would avoid duplicating slashes, and make sure this also works on non-unix systems. For urls, there doesn't seem to be a standard library solution (urllib.parse provides some functionality but only works with full urls as far as I can tell).