List Sentry Errors in GitLab

We should make it easy to browse the list of errors in a configured Sentry server, from within GitLab.

To do this, we should:

1. Add a new Operations->Errors sidebar entry, which goes underneath Tracing
2. When opened without configuring Sentry, we should present an empty state describing the feature and linking to the Settings page
3. When Sentry has been configured, we should display a list of the errors from Sentry. Design should follow general best practices from Sentry, still TBD:
4. Clicking on an error should then pop open a new tab, directly to the Sentry URL for that error, until a dedicated view is implemented.