[E-mail] Create a unified e-mail sending service

[tudoramariei]

Is your feature request related to a problem? Please describe.

E-mails are being sent from both the back-end and the infrastructure side of the project.
Currently, if an e-mail template is changed in the back-end, some e-mails will stay the same, since they are sent directly through Cognito.
There should only be a single source of truth regarding e-mails so that fixing them in one place works across the whole platform.

Describe the solution you'd like

Move the e-mail templates to or from code4romania/onghub.