Make commit metadata CI check less annoying

Currently, there is a CI check that makes sure that each commit has metadata. This is YAML metadata at the end of the commit that includes the type of the commit. For example: `type: documentation`.

There is a script that automatically generates a [changelog](https://imdl.io/book/changelog.html), and it puts an emoji by each commit that represents the type of the commit. So, I wanted to make sure that I didn't forget to annotate commits.

However, this is unusual and, I think, annoying for contributors, who aren't used to this requirement. Also, the check fails if any commit in a string of commits don't have metadata, even though those commits are all going to get squashed together.

One idea is to change this to only check commits which are signed by my key, since I sign all commits with my key before merging, that would make sure that no commits without metadata make it into master.