PR previews should use test database

[tobiasdiez]

Currently, the azure pr preview uses the production app settings, in particular the main database.
Changing this requires one of the following to be fixed:

• Pre-Production environments use same settings as production as default Azure/static-web-apps#86
• staticwebapp appsettings set: Support environments Azure/azure-cli#22552
• Add ability to set app-settings-json similar to github action azure/appservice-settings@v1 Azure/static-web-apps#677

Using

      - name: Login to Azure
        uses: Azure/login@v1
        with:
          creds: ${{secrets.AZURE_CREDENTIALS}}

      - name: Update app settings
        uses: azure/CLI@v1
        with:
          inlineScript: |
            az staticwebapp appsettings set --name jabref-online --setting-names "DATABASE_URL=$DATABASE_URL"
        env:
          DATABASE_URL: ${{ secrets.AZURE_TEST_DATABASE_URL }}

Alternative: #1343

[tobiasdiez]

Fixed with #1343.