Fix broken new store form

Closes #12 (closed)

What does this MR do?

The new store page was broken, seemingly due to combining two code paths in another change.

Most of the issues actually are just warnings about missing keys that Whoops has a big fit over.

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

Not entirely sure yet, still WIP... not sure which other pages are impacted, although I'm making quite conservative changes generally.

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))