created new class MaintenanceGateway

Treats #9 (closed)

What does this MR do?

Starts refactoring Maintenance to Gateway. Deleted unused and dead code.

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

@NerdyProjects could you take a look if the delete part in the Gateway is correct? If it is, I am confident.

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))