Add mailer

[ylaifa]

Why ?

Sending an email is a good way to inform our users .

Must Have

• [x] Config action_mailer to development environment
• [x] Config action_mailer to production environment (Sendgrid)

Reading list

• https://www.thehackingproject.org/dashboard/courses/1/weeks/6/days/1?locale=fr
• https://guides.rubyonrails.org/action_mailer_basics.html
• https://app.sendgrid.com/guide/integrate/langs/smtp