ref(grouping): Move grouping config to top level of grouping info

[lobsterkatie]

This changes the format of the grouping info API response, so that instead of the full serialized grouping config being included in every variant, only the grouping config id included at the top level of the JSON, and the variants in turn are included under a new variants key. In other words, it changes from a response that looks like this:

{
  app_exception_stacktrace: {
    ...,
    config: {
      id: "newstyle:whatever",  // the only part of `config` we actually care about
      ...
    }
  },
  system_exception_stacktrace:  {
    ...,
    config: {
      id: "newstyle:whatever",  // we don't need this twice
      ...
    }
  }
}

to one which looks like this:

{
  grouping_config: "newstyle:whatever",
  variants: {
    app_exception_stacktrace: {...},
    system_exception_stacktrace: {...}
  }
}

This not only removes redundancy (along with a bunch of information about the config we don't need), it also sets us up to be able to send other grouping-info-wide information in the response.

Note: Before this can be merged, the front end needs to be adjusted to work with both the old and new formats. Once that's deployed, and this is merged and deployed, we can remove old-format handling from the front end.