+- Configurable subjects for all emails send by astakos app. Introduced settings
+ * ASTAKOS_INVITATION_EMAIL_SUBJECT
+ * ASTAKOS_GREETING_EMAIL_SUBJECT
+ * ASTAKOS_FEEDBACK_EMAIL_SUBJECT
+ * ASTAKOS_VERIFICATION_EMAIL_SUBJECT
+ * ASTAKOS_ADMIN_NOTIFICATION_EMAIL_SUBJECT
+ * ASTAKOS_HELPDESK_NOTIFICATION_EMAIL_SUBJECT
+ * ASTAKOS_EMAIL_CHANGE_EMAIL_SUBJECT
+ * ASTAKOS_PASSWORD_RESET_EMAIL_SUBJECT