Reloading the module shouldn't delete associated mail templates

[atm-florianm]

Summary
Currently, when you restart SellYourSaas (for instance after upgrading it), you lose any customizations you made to the module's mail templates (fortunately, the daily SQL dump can save us).

To Reproduce

1. Go to Home > Setup > Emails > Email templates
2. Modify a mail template, for instance InstanceDeployed
3. Disable SellYourSaas, then enable it again
4. Go back to Home > Setup > Emails > Email templates, scroll to InstanceDeployed (for instance)
5. The modifications you made are lost

Expected behavior
Enabling the module should not delete (or reset) previous customizations made in the module, including e-mail templates.

[eldy]

Fixed by #310