insights: change series_id to string (hash); add index

Created by: slimsag

Today, series_id will need to be some unique identifier of the actual series definition in the user/org/global settings, e.g. there are two unique series here:

"insights": [
    {
      "title": "fmt usage",
      "description": "fmt.Errorf/fmt.Printf usage",
      "series": [
        {
          "label": "fmt.Errorf",
          "search": "errorf",
        },
        {
          "label": "printf",
          "search": "fmt.Printf",
        }
      ]
    }
  ]

The plan was to have series_id effectively be a hash of the source of settings (user ID / org ID / global) + the actual info associated with the insight series. However, an int32 is likely not sufficient for this due to collisions. I considered using crc64 with an int64 data type, but there is still some uncomfortable room for collisions there and text is cheap - so let's just store text for now and use a sha256 hash to err on the side of caution.