add default logo and loading previewer logo svg or png

[ghost]

The same as the previewer page. A default image was also missing.
This first checks whether svg or png exists, if this is not there then a default image is loaded.

[qqmyers]

Looks good - thanks

[qqmyers]

@BenediktMeierUIT - I think I merged this too quickly. It looks like the change to add internationalization #19, made the fileupload2.js script a module which interferes with being able put your handleImageError method in as an onerror function. I pulled that method out into a separate js and avoided sending the siteUrl variable to it to make things work again in the main branch ( see 3be8983 and 01935a8 ). If you know of a cleaner way to do this, please feel free to add a new PR.

[ghost]

Okay, I'll take a closer look. I will change something there. Thanks for the reference.