Add an option to set the default Dashboard image

[lzlrd]

I think it would be great to be able adjust the default background of Dashboard for new users. In my case, I think it looks great just set to the default colour, but of course different administrators would vary on that.

I believe an option would be quite useful especially for companies, schools, and other uses where Nextcloud is to be branded. This could help them buy allowing the Dashboard to have a branded background image (as they're likely not going to see the login page much).

A possible extension is to also deny the changing of Dashboard's background in the case where branding is necessary (is it ever, though).

Is your feature request related to a problem? Please describe.
No

Describe the solution you'd like
I take it that we love the config.php file here and well, since Dashbaord merged into server recently it seems fair enough to place the option there.

Describe alternatives you've considered
I don't think adjusting it for every user is a fair alternative.

Additional context
Maybe defaulting to the background colour would also be preferred over the "clouds" image that is currently the default.

[kesselb]

Search ;)

[lzlrd]

My apologies, I completeley missed #23558.