Added support to move to the next unannotated document

[josepablog]

• Created fast-forward icon for client side
• Added server side logic to find next document that does not have an annotation
• Added client side logic to query server to find document without annotation, and to jump to it

[josepablog]

I have a library with a lot of documents on which many users are contributing annotations. Current interface makes very cumbersome to find the next document.

This pull request fixes this.