Add default icon for assets with invalid icon

[akien-mga]

Typically a Godot logo... though we could get one of the artists in the community to make a custom icon, potentially based on the logo, but that would clearly identify a "default" icon.

[TomWor]

Just a note: Right now, the icon is just a URL provided by the asset author that doesn't get fetched into the asset library, but only put into the img src attribute:

https://github.com/godotengine/asset-library/blob/master/templates/assets.phtml#L72

To enable a default asset icon, the asset submission form needs to be updated to enable uploading an asset icon to the asset library application - so there can be a file-check before rendering the img tag.