Correct return values in Database insert/insertMultiple functions

Closes #916 (closed)

What does this MR do?

• Database:insertMultiple now returns the number of altered rows instead of the index of the first one
• Database:insert keeps returning the index of the inserted or updated value

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

Very sure. I simply copied the code. The return value of insertMultiple isn't used anywhere yet.

How to test

Check that any action which writes or updates data still works.

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• used "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• added a short text that can be used in the release notes
• Once your MR has been merged, you are responsible to create a testing issue in the Beta Testing forum: https://foodsharing.de/?page=bezirk&bid=734&sub=forum
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...) how this change can be tested.
  • Be aware, that also non technical people should understand.

Release notes text in German