Add max_depth limit to GraphQL queries once again

We accidentally removed this code in commit c984e666f. As mentioned in
our GraphQL documentation, limiting the depth of the queries helps
against DoS attacks.

app/graphql/consul_schema.rb

@@ -1,4 +1,6 @@
 class ConsulSchema < GraphQL::Schema
   mutation(Types::MutationType)
   query(Types::QueryType)
+
+  max_depth 8
 end

spec/graphql/consul_schema_spec.rb

@@ -0,0 +1,37 @@
+require "rails_helper"
+
+describe ConsulSchema do
+  let(:user) { create(:user) }
+
+  it "returns an error for queries exceeding max depth" do
+    query = <<~GRAPHQL
+      {
+        user(id: #{user.id}) {
+          public_proposals {
+            edges {
+              node {
+                public_author {
+                  username
+                  public_proposals {
+                    edges {
+                      node {
+                        public_author {
+                          username
+                        }
+                      }
+                    }
+                  }
+                }
+              }
+            }
+          }
+        }
+      }
+    GRAPHQL
+
+    response = execute(query)
+
+    expect(response["errors"]).not_to be nil
+    expect(response["errors"].first["message"]).to match(/exceeds max depth/)
+  end
+end