Support custom email templates for notifications

Problem to solve:

Customers migrating from other CI platforms (namely Jenkins) have built custom email notifications for pipeline events (build success, build failure, etc). They customize the HTML content of the email itself, and include attachments (ie: build logs). This information is attached so that certain audiences do not need to purchase licenses for their CI platform to get relevant SDLC information.

However, when migrating to GitLab, they cannot customize our Pipeline Status emails, and these users lose visibility into the SDLC.

Requirements for solution:

• GitLab should allow customization of the Pipeline Status email content