Support S/MIME signed commit signatures

After pushing a S/MIME signed commits using smimesign to GitLab, the UI shows An error occurred while loading commit signatures on top of the page. Also, verification badge pinwheel next to commit hash keeps spinning forever, for every commit in the commit history.

Ideally, GitLab should be able to verify the S/MIME signed commit (or) at the least, show unsupported signature.

To reproduce the issue

1. Setup git to use smimesign utility
2. Make a commit with -S option
3. Push code upsteam
4. Open the URL for the repository

Example GitLab repo https://gitlab.com/ven/smime_test/commits/master