changed icons on statistic-page from png to font awesome svgs

Closes #538 (closed)

What does this MR do?

Changed icons on statistic-page from .png to font awesome svgs and added some icons as mentioned in the issue comments.

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

Very confident. No test is added, because no function is integrated.

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))