refactor: moving router and api of sessions & users into own packages

[mateo-ivc]

Description

Moving routes and api files into own packages (user/sessions) based on PR#5510

Changelog

• expand initializer package by api- & routes initializer
• moving api.Context.go into own package -> middleware
• moving middleware that need a service into designated package
• each package has not it's own router -> will later be joined together to a single big router
• added api tests

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• The light- and dark-theme are both supported and tested
• The design was implemented and is responsive for all devices and screen sizes
• The application was tested in the most commonly used browsers (e.g. Chrome, Firefox, Safari)

(Optional) Visual Changes

[github-actions]

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