JOINDIN-315: Add Flickr machine tags functionality

This continues on from Tocacar's PR (https://github.com/joindin/joindin-web2/pull/26), and merges Master into the branch, fixing conflicts. I've also fixed the frontend, which originally used the old one-page event page

When merging in, we'll need to create a flickr API and update the config on the server, and add the following tickets to Jira to make it more stable:
- Do something if/check if Flickr is down (the page loading is currently reliant on flickr being up)
- Consider a non-js fallback for loading the photos