Allow multiple users to sign the same document

More than one user should be able to sign the document.
In that case, we should let only one picture and the name and date of all the signers one below the others.
== FROM the duplicate task: NXP-6700
Allow for adding multiple signatures to a document
This will require:

• modificaitons of signature entry forms
• adjustments to signature watermarks (formatting, size)
• changes to signature application logic (verificaition if a document was signed by a particular CN, not - as it is now - if signed at all)