Feature/pt clean up api

What does this MR do?

refactores the API module and transfers model to gateway as asked in #9 (closed).

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

if tests pass, I am confident. there have been no actual changes.

Checklist

• added a test, or explain why one is not needed/possible... (tests work and it is 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))