When a project is created, add it's default branch to the vulnerability tracked refs list by default

When a project is created for the first time, it is likely expected that vulnerabilities detected on the default branch would like to be tracked. As such, we'll add the default branch to the tracked refs list by default.

This may, however, not succeed, if the organization that owns the project has reached it's tracked refs quota, in which case the branch will not be tracked until the user frees up quota space or requests that it's increased by GitLab. This should ideally be very visible to the user.