Resolve "Legal use of map tiles"

Closes #91 (closed)

What does this MR do?

Change the tile map source from ArcGIS to Wikimedia, which is free to use

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

Very. Only four lines were changed and everything seems to work.

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))