provide a dropdown to join or remove empty slot

What does this MR do?

• Provides an optional dropdown for empty pickup slots:
  • join pickup
  • remove pickup slot

that is shown, when both options are available (e.g. for the last empty slot, when the coordinator is not already part of the pickup)

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

quite

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))