Reduce number of requests in SubmissionShow

In `SubmissionShow`, we request the OData for the submission, as well as the submission's first version. The only properties we use from the OData are `__id`, `__system`, and `meta` (for the instance name). The only information we use from the first version is the user agent. Now that getodk/central-backend#783 is in place, I think we could probably consolidate these two requests. I think if we request just the logical submission, that will include all the information we need, including the first user agent and the current instance name.