Configure user defaults (#2490)

* new users inherit defaults from config-user.php

* installer creates ./data/config-user.php

* fixed typo

* .gitignore fix

* fixed style issues

* Fixed comments

* Update according to feedback

- rename file into `data/config-user.custom.php`
- make it optional (and so, don't copy it during installation)

* fixup! Update according to feedback
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