The 21travel realm has been prepared for import
as follows:

 - selected 21login theme
    - Realm settings -> Themes -> Login theme -> selected 21theme
 - marked redirect URI value to be changed by customers
    - Clients -> account -> Valid redirect URIs -> changed to "/replace/with/your/url"
 - added 21travel audience to access tokens
    - Client scopes -> Create client scope -> Name: "21travel-audience", leave the rest -> Save
    - Mappers -> Configure new mapper -> Audience -> "Included Custom Audience": "21travel", leave the rest -> Save
    - Clients -> account -> Client scopes -> Add client scope -> Add created scope as default

With these things prepared, the only thing left to do
for customers is to set a valid redirect URI. This will
be documented in an accompanying MR against the docs.

In the future, one may consider doing this automatically
with the help of the REST APIs provided by both Caddy and
Keycloak.