usermanager CLI: catch plugin signalled error messages

Plugins may prevent user modifications and will use msg() to tell why.
This will catch the messages and print them on CLI as well

abort early when no auth backend is available

type hints and cleanup

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typos

remove unused variable

add addToGroup and removeFromGroup

initial usermanager cli

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Consistent capilization of io_saveFile and psr-2 formatting

jquery-migrate: replace on()/trigger() shorthand and use prop() for disabled attr

Related to #1546.