Implement functionality for sending mails to group's members

Functionality for sending letters to group's members should be based on parent User Story POULPE-443:
After pressing "Send" button in the form for sending mails(see POULPE-444) next actions should happen:

• The new letter(message) should be created. This letter should have topic and body specified in the mail form.
• The sender's e-mail in the letter should be equal to e-mail of current admin (admin, that sends letter).
• The created letter should be send to all users, that belong to selected group(group, we send message to).
• After sending mail to all members the message "Your message was successfully sent!" should appear on the screen.