Merge request title only option

Merge request events are great to have coming into a mattermost chat. The issue is if you have a default template or really any template in the body of the merge request, it gets outputted to the channel every time. This request is to capture the merge request event but only print out the title and not the body. I would think that it would be another subscription option

Start by tracing how merge request events are subscribed to and how their messages are rendered for Mattermost. Confirm whether subscription settings already support event-specific output choices; done means a merge request notification can include only its title while preserving the existing full-body behavior for other configurations.

Written by the indexing model from the issue text.