Add possibility to reactivate abandoned stores

Closes #405 (closed) and #209 (closed)

What does this MR do?

This is an attempt to solve situations in which there's no BIEB left in a store which at the same time makes it impossible to add a new responsible BIEB.

This MR adds a check that determines if there's a store manager left in store. If that's not the case, instead of the usual FS picker a BIEB picker is shown (together with a warning message) that allows ambassadors to select up to three new store managers for this store.

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

Confident.

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. Checkout branch locally
2. Login as Bot
3. Create a store
4. Remove responsible FS in DB
5. "Team bearbeiten"
6. Should display a warning + allow to add new store managers

Screenshots (if applicable)

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))
• Once your MR has been merged, you are responsible to update the #foodsharing-beta Slack channel about what has been changed here. They will test your work in different browsers, roles or other settings