WIP: Weekly rotation of landing page splash image

What does this MR do?

This MR removes the old campaign with walkingbread, faulesei, sauregurken, foodporn etc. and replaces it with a new campaign of splash images. For now it is just 3 placeholder images supplied by @em.ka that are rotated in a random fashion. More images will be added during the next months, until eventually we have 53 images in total, one for each week. Then we will randomly show either the current week image or one of the previous 4 week images.

How confident are you it won't break things if deployed?

I tested it in Firefox 61, not any other browser. It won't break the site, but needs to be tested for cross-browser compatibility. I did my best not to include any problematic css.

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))