Store pickup api

closes #51 (closed)

What does this MR do?

• Moves addFetcher to rest api
• have a store service to find out about available pickup slots
• Adds foreign keys for pickup handling tables to store which should be applied on production as well
• implement proper logic to only allow signing up for
  • existing pickup slots
  • pickup slots the user has not signed up for already

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

needs testing but feels quite safe

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))