WIP: Photo thumbnail on dashboard

Closes #454 (closed)

What does this MR do?

Images are displayed as small thumbnails on the top-left of dashboard posts. Clicks lead to the wallposts-section on the specific user page.

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

Pretty sure. Didn't really touch anything too terrible.

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))