Add basket counters to statistics

Partially treats #81 (closed)

What does this MR do?

Adds a counter for total number of baskets shared and average weekly baskets shared.

http://localhost:18080/statistik or https://beta.foodsharing.de/statistik

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

pretty sure. However we have to check if I used the fs_basket database correctly.

Links to related issues

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))