feat Notification (email function)

[anna-sara-maria]

• Fixed the last things on the send_email() function and integrated it into the server class.
• send_email no longer takes receiver_email as an argument, but rather has it stored as a string inside the function.
• the notify() function no longer had any purpose so I deleted it

[anna-sara-maria]

Seems like the code has already been integrated into the main branch!