Untranslate mass_mailing subject
CHANGELOG:
### FIXED
- [#820](https://trello.com/c/rBZH8SRo/1272-error-assumpte-mass-mailing) Untranslate mass_mailing subject
https://trello.com/c/rBZH8SRo/1272-error-assumpte-mass-mailing
Related card:Edited by Pelayo García