move custom.css from css to config

[khassel]

This is another change to cleanup structure, already mentioned in #4019 (comment)

After separating default and 3rd-party modules this PR moves the custom.css from the mm-owned directory css into user owned directory config.

It has a built-in function which moves the css/custom.css to the new location config/custom.css (if the target not exists).

Let me know if there's a majority in favor of this change.

[KristjanESPERANTO]

Perhaps instead of automatically moving it, outputting a deprecation warning if custom.css is still in its old location? This would have the advantage that users would have to do this consciously by themself.

[khassel]

I don't care at all; I've implemented the most convenient solution for now.

Let the majority decide.

A warning is already displayed; you can change the text, abort with an error message, or...

[sdetweil]

I’m ok with moving it. And moving if detected in old place