[feature/accessibility] Provide alt text for default avatars + headers

[tsmethurst]

We should provide some alt text for the default GoToSocial avatars and the default blank header, so screenreaders have something more useful to go with than just "header for xyz user" or "avatar for xyz user", which doesn't really say anything at all about the image content. Since we don't store attachments for those default images we'll have to think of a nice way of doing that. Since all the default avatars depict basically the same thing (a cute lil sloth sort of greyed out) we can probably use one description for all avatars.