Easy user switching

This feature-request is closely connected to [my request](https://github.com/wallabag/wallabag/issues/2989) to main wallabag.
While it is still useful to have mass moving on from the admin UI, it is also cool to add individual articles to certain users on-demand.
What I propose is adding multiple user to Wallbagger settings UI and easy switching between them.
Now we have only single user credentials

It would be nice to add multiple users here which can be switched, e.g. via radiogroup or another way.

Start at the Wallabagger settings UI and trace the existing single-user credential flow. Determine how multiple users could be stored and selected, then verify that switching users changes the account used for article actions and that the existing single-user behavior remains intact.

Written by the indexing model from the issue text.