Fix first query in GEL query needing to be GEL

[kylebrandt]

Currently, using the datasource backend plugin api route in grafana /api/tsdb/query only looks at the first query to decide what the datasource is.

Since GEL is multi-datasource, the logic is that it is a "GEL query" if any of the queries are GEL. But currently GEL only works if the first query in GEL is a "GEL query" (Math or Reduce - "Add GEL" in the UI).

The high level options are:

1. Some sort of hack:
   1. Change the /api/tsdb/query endpoints to look at all the datasources, and chose GEL if it is any of them.
   2. Change the ordering of queries so a GEL query is always first in the front end.
2. Create a new query endpoint for GEL

I think creating a new endpoint is probably the most sane. Both the "sort of hack" have the issue that if when using the GEL datasource, you only have datasource queries within the GEL query (and no math/reduction etc) that there won't be any GEL nodes to allow the endpoint to see that it is GEL (unless we always put some sort of dummy node first).

The is also a related issue with the response type of Frames, when there were no Frames added to the response (will create another issue, but this also points to another api endpoint for me).