♻️(backend) change email invitation content

Change the email invitation content. More
document related variables are added.
To benefit of the document inheritance, we moved
the function email_invitation to the document model.

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to
 ## Changed
 
 - 💄(frontend) error alert closeable on editor #284
+- ♻️(backend) Change email content #283
 
 ## Fixed