Pt replace prefix

Closes issues0#421

What does this MR do?

replaces the keyword PREFIX in the whole code base.

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

Should be ok.

Links to related issues

issues0#344

Checklist

• added a test, or explain why one is not needed/possible... (only refactoring)
• 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))