ui: Move validation button for workflow items to workflow item table

[mathiashoeld]

Currently, the document validation button is hidden within the workflow item details. Instead, a button (for example paper clip symbol to represent "attachment") could be introduced on the right hand side of the workflow item showing in a more prominent way that a document needs to be validated before closure of the workflow item. This paper clip should only appear if there is a document to validate. Clicking the paper clip should show the document name, who uploaded it and when, and allow to validate the document.

[mathiashoeld]

To keep a lean design, the validation button should not be on the workflow item table. BUT: If there is a document added to the workflow item, there should be some kind of symbol that marks the existence of a document (e.g. a paper clip symbol).