Turn KitchenOwl into a monorepo

[beatbrot]

Second try of #351

Fixes:

• Deleted all backend tags
• Correctly link to issues/PRs in old repo
• Do not relocate .github
• Fix path in pytest action

[beatbrot]

@TomBursch Alright, I fixed all remaining issues :)
Here is what I did to make this move:

1. Install git-filter-repo
2. Checkout and cd into kitchenowl-backend
3. Create a file commit-msg.txt outside the repo with the content #==>TomBursch/kitchenowl-backend#
4. Run in the repo python git-filter-repo --filename-callback 'return filename if filename.startswith(b".github") else b"backend/" + filename' --replace-message ../commit-msg.txt
5. Push those changes to a git repo (NOT the new main repo)
6. Checkout and cd into main repo and create a new branch
7. Add the repo from step 5 as remote
8. Call git merge --allow-unrelated-histories tempremote/refactored
9. Done!

Now, I guess it's your turn :) Feel free to do with this PR whatever you want. I can re-do all those steps anytime so that we have the latest histories at all time. But of course you can also do that yourself.

[TomBursch]

@beatbrot I've moved the flutter app to the KitchenOwl subfolder. I know the naming is not great, but I'm not sure if having the folder name be different from the app name is supported by flutter.

I've also added the Dockerfile for the combined image 🎉