✨(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

CHANGELOG.md

@@ -20,6 +20,7 @@ and this project adheres to
 
 - ♿(frontend) improve accessibility:
   - ♿️(frontend) fix subdoc opening and emoji pick focus #1745
+- ✨(backend) add field for button label in email template #1817
 
 ### Security