Werk #309: Trying to prevent read/write conflicts with a large user base

When having a large amount of users, it might happen, that the user configuration files are read while they are written e.g. by an automated LDAP sync or some other modification to the user database. With bad timing, the file could be read while being incomplete which might result in syntax errors or similar unexpected situations. This changes the way the files are being written to a more save way.

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