Aggregate Errors from Error Tracking

Building on top of the MVC of error tracking (https://gitlab.com/gitlab-org/gitlab-ee/issues/5686), we should expand support to include aggregation of similar errors.

This is a key capability, as otherwise frequently logged errors could quickly overload the error tracking system and larger issue board of GitLab. This is not only a UX problem, but until the system can understand and aggregate errors it also won't be able to determine other key attributes:

1. How often an error occurs
2. If an error is new (they all are new!)
3. If an error has been resolved
4. and more.