Add backend for DSVGO banner settings

[davisagli]

https://gitlab.kitconcept.io/kitconcept/distribution-kitconcept-website/-/work_items/46

This adds a control panel for configuring settings for the @kitconcept/volto-dsgvo-banner add-on. This PR is a draft because the frontend add-on still needs to be updated.

What's included

• registry settings (with same defaults as the existing Volto settings in kitconcept/volto-dsgvo-banner)
• dsgvo-settings control panel (for REST API / Volto)
• ISiteEndpointExpander to include the value of these settings in the @site endpoint
• Upgrade step
• Functional tests
• Fix locales (kitconcept.website domain rather than kitconcept.site, add missing DE translation)
• Fix default language setting (remove it from the default profile so it doesn't get overwritten during an upgrade)

To do

• Use a new release of volto-dsgvo-banner
• Docs (there's no infrastructure for this in the website distribution yet))

Screenshot

[Tishasoumya-02]

PR in dsgvo addon: kitconcept/volto-dsgvo-banner#55

[sneridagh]

@Tishasoumya-02 merged! Please continue with it.

@davisagli we don't add for now the ID of the tracker? I guess it's the next thing that we will need to customize.