Allow configuring email templates to use graphical assets from external source

Problem Definition

When using external mail service providers (like Gmail, Outlook 365) graphical assets won't be displayed in email messages if Bitbucket is not exposed to the internet, as the source of an image is the Bitbucket Server itself.

It will just display like that:

Suggested Solution

Offer publicly accessible graphical assets repository for email templates as an alternative image source in email templates.