✨(backend) add field for button label in email template (#1817)

## Purpose

The email template is made with the idea that they link to a document.
This change allows to customize the label of the button (currently,
"Open") to allow for a different action verb. Additionally, the
'document_title' parameter is renamed to 'link_label' to reflect that it
can link to other things than documents.

## Proposal
- [x] Email template `template.mjml` updated as proposed
- [x] Method `send_email()` updated
- [x] Translations updated

src/backend/core/models.py

@@ -1,6 +1,7 @@
 """
 Declare and configure the models for the impress core application
 """
+
 # pylint: disable=too-many-lines
 
 import hashlib
@@ -824,7 +825,8 @@ class Document(MP_Node, BaseModel):
                 "document": self,
                 "domain": domain,
                 "link": f"{domain}/docs/{self.id}/",
-                "document_title": self.title or str(_("Untitled Document")),
+                "link_label": self.title or str(_("Untitled Document")),
+                "button_label": _("Open"),
                 "logo_img": settings.EMAIL_LOGO_IMG,
             }
         )