Epic: email notification improvements

[susilnem]

Description:

Setting up this ticket with the improvements that we have to implement in the email notification.

• Remove hardcoded URLs
• Use static_full_path for static files used in email templates example: Fix image issue on dref notification #2489
• Refactor send email functionality (notifications/notification.py)

[szabozoltan69]

" Remove hardcoded URLs" is done and deployed to Staging.