Add X-GitLab-Subject header to notification emails

Description

It would be lovely to have a way to discriminate email notification types; "commits", "tags", "branches", etc. in a more automatable way.

Concrete use case: I'm a dev at a company. Though not all information generated on our GitLab instance is relevant to me, there is still the need to have an eye on what is going on in a broader sense. For this, there are things like for instance pipelines that have a lesser importance in case of success, yet are more important when failing and both cases overall less important than commits. But they are of relevance never the less, even if it is to have a feel for where programming hours are going. Commits are important since it is also my job to fulfil the role of stewardship on some projects. List goes on and on. But currently there is no clean way to tell the filters of an email client to move a particular email to a particular folder. You have to parse the subject. This works well based on keywords "branch", "tag", "Pipeline" (note the inconsistency), ...and then comes commits. Though any amount of multiple commits within a single push appear as "commits: ", once you have a push with only one, it becomes a matter of filter precedence to further filter out commits from discussion answers etc.

Arguably one could say, well your notification settings are just to verbose. As an owner of various active repositories it is of my personal preference to have all information available before filtering/juggling/sifting it, instead of silencing something that can and occasionally is of relevance.

Proposal

Add a X-GitLab-Subject header to denote/tag/discriminate the genre or subject of the email. Whether it is an "Email", "Build", "Issue", "MR", "Discussion", "Pipeline", "Commit". This allows email clients to sort the emails into individual folders based on the category of content they carry, thus enabling the developer to better prioritise as well as keep in touch with the information stream generated by the GitLab platform.

Links / references

As reference there is the already existing X-GitLab-Project*, X-GitLab-Pipeline*, X-GitLab-MergeRequest* headers.