Add API endpoint to get the group for a form

[stephencdaly]

What problem does this pull request solve?

Trello card: https://trello.com/c/CsEqMtDL

Add an API endpoint that returns the group details for the form with the given ID. This includes the group admin users, and the organisation details including the organisation admin users as embedded resources.

This endpoint will be used by forms-runner to get the group and organisation details for the form and the contact details for the group and organisation admin users in order to send email notifications when submissions bounce.

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[lfdebrux]

I think what was there before was correct, based on the other request specs in this repo, and the template in rspec-rails

Suggested change

	describe "GET #show" do
	describe "GET /show" do

[stephencdaly]

Hmm actually I think neither are consistent with how we usually do it elsewhere.
Looks like we do describe "GET /path" or describe "#action"

[lfdebrux]

Oh yeah you're right /show is an action not a path