Improve the avatar management pop up

[aethior]

I propose some improvements:

• Centre the avatar image in the middle of the pop-up instead of aligning it to the left.
• Instead of the “Do not use avatar” button, add a “Delete” button and display the default avatar or a blank image when the avatar is deleted (more modern and in line with how other websites do it).

[BurntimeX]

I agree that it looks better. However, the standard FormBuilder component for uploads is used here, which does not support this type of layout. I don't think it's worth building a separate component just for this use case.