✨(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/mail/mjml/template.mjml

@@ -22,7 +22,7 @@
           <!-- Main Message -->
           <mj-text>
             {{message|capfirst}}
-            <a href="{{link}}">{{document_title}}</a>
+            <a href="{{link}}">{{link_label}}</a>
           </mj-text>
           <mj-button
             href="{{link}}"
@@ -30,7 +30,7 @@
             color="white"
             padding-bottom="30px"
           >
-            {% trans "Open"%}
+            {{button_label}}
           </mj-button>
           <mj-divider
             border-width="1px"
@@ -56,4 +56,4 @@
       </mj-section>
     </mj-wrapper>
   </mj-body>
-</mjml>
+</mjml>