API: move create/delete user calls to usermanager

This only moves the calls. A proper refactoring of the user manager
would make sense:

1) introduce a helper component covering the basic operations

API: move create/delete user calls to usermanager

This only moves the calls. A proper refactoring of the user manager
would make sense:

1) introduce a helper component covering the basic operations including
proper error signalling using Exceptions
2) refactor admin and cli components to make use of 1)
3) make the operations in 1) available via the API

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use $auth instanceof AuthPlugin instead of not null check

Rector to rename print to echo calls

code style: ptln removed, useradmin cleanup

coding style: control flow line breaks

coding style: control flow whitespaces

update some plugin classes in phpdocs, reformatting

recover comments in list

codestyle adjustments: function declaration braces/spaces

Apply rector renames

Apply remaining rector fixes to lib

Apply rector fixes to the rest of lib/plugin

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add needed type hints for phpunit8

This will break a lot of plugin tests, but can't be avoided

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