New backoffice web UI as blazor/razor pages

[Metawolve]

Currently, the backoffice for the backend API is a react app, currently available here:
https://app.eurofurence.org/backoffice/#/

The current github repository (archived) is:
https://github.com/eurofurence/ef-app_react_backoffice

This should be integrated into the backend using Blazor or Razor pages - which one is easier/better has to be evaluated further.

Most features from the current backoffice can be re-created as-is in the new backoffice.
Some features can be left out for now, like the fursuit badges.

Other features should be simplified, like the image upload.
We don't need reusable images where the user has to provide the ID anymore. Instead, it should be possible to just upload the image in the relevant form, e.g. for a knowledge base article.