fix: add extra context to template in marketing app

Created by: arjunsinghy96

Adds extra context base_url, signature_name and signature_title to the context of email templates.

Related Tickets

• BB-4368

Testing Instructions

Including the testing instructions relative to stage OCIM server for easier testing.

1. Redeploy this branch to stage if not already deployed.
2. Visit templates admin page
3. Select template with id 2 ( BB-4368 Test template )
4. Select Send sample option and click Go.
5. Check you email inbox for received email and verify that the email does not have broken images.