Ability to customise the default avatar for the instance

Is your feature request related to a problem? Please describe.
In order to improve the corporate identity of a facility, it would be necessary to be able to customize the default avatar for users. This avatar appears when an official meeting or proposal is created, and there is no way to customize it with the organization's icon or logo.

Describe the solution you'd like
Just like in the General Configuration / Appearance section, where you can customize the icon, organization logo, etc., you could also customize the organization's avatar.

Describe alternatives you've considered
You could also use the image posted with the icon with the avatar of the official contributions.

Additional context -

Does this issue could impact on users private data? No

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