Refactor frontend URL and redirection logic into its own module

Motivation

Other modules building similar solutions, like https://www.drupal.org/project/transform_api → , could you re-use the frontend URL handling parts of lupus_decoupled_ce_api.

Proposed resolution

* Factory URL logic out into a separate sub-module
* Keep lupus_decoupled_ce_api expose the /ce-api endpoint and make it focused on that.

Remaining tasks

* Find good module name. Maybe just "lupus_decoupled_frontend" module, since it handles the relationship to it.
* Decide what to include. Is it ok to have UI in there? Where to move the UI else?