Refactor: Simplified is_admin determination

[onefloid]

Determining whether a user is an admin based on the name is is simplified now

• Access to the _user property instead of multiple accesses kwargs.get('user')
• Use only one if statement with two conditions instead of multiple if statements
• "Default" value None moved to the top
• Removed method _determine_is_admin because the result was always overwritten afterwards
• Less code

Tests were passed successfully

============================================================= test session starts =============================================================
platform win32 -- Python 3.11.7, pytest-8.1.1, pluggy-1.4.0
rootdir: D:\dev\tm1py_onefloid
configfile: pyproject.toml
plugins: anyio-4.2.0, xdist-3.5.0, time-machine-2.13.0     
...
========================================== 816 passed, 36 skipped, 198 warnings in 981.11s (0:16:21) ==========================================