If applied, this commit will make so that when a user uses the layout of a template document on a new document, the radio buttons will stay linked.
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Using the layout of a template document, containing radio buttons, on a new document will unlink radio buttons, making them useless.

1. Go to the Sign module, and click on the Templates tab.
2. Click on the "Upload PDF" purple button, and upload a PDF of your choice.
3. On the left side of the screen, there's a panel where we can drag and drop sign items. Drag and drop a Radio button. 2 buttons will automatically show, and a yellow link will appear between the buttons. This link indicates that, when a user opens the document to sign, when clicking on a radio button, the other one will automatically be unselected if previously selected.
4. Save and return to the Templates window.
5. A new row has appeared with the name of the document. On the right side of this row, hover the house next to the share button and click on the 3 dots. Then, click on Use Layout.
6. Upload a PDF of your choice.
7. When the PDF shows, there's no yellow link between the radio buttons, unlike on the template from which the layout was used.

In the file sign/models/sign_document.py, in the method _copy_sign_items_to(self, new_document), we copy every sign item from the template to the new document. However, the copy method we call doesn't take into consideration the special link between radio buttons, replicating the buttons correctly but not the link. This link is expressed by the radio set to which the radio buttons belong: indeed, after the copy of sign items, each radio button belongs to a different radio set, separating them.

 ### Explanation of the fix
The fix consists of a rework of the _copy_sign_items_to method and a brand-new method specifically used to copy radio buttons. To keep the link between them, we assign all the buttons of the same set on the original documents to the same brand new radio set. The first button is copied normally: this creates a new radio set. Then, for the remaining buttons, we call copy_radio_item, to which we pass the button created first to use the information the next buttons will share with it.

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 opw-4842590

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