Extract class to manage GeozoneStats

Even if this class looks very simple now, we're trying a few things
related to these stats. Having a class for it makes future changes
easier and, if there weren't any future changes, at least it makes
current experiments easier.

Note we keep the method `participants_by_geozone` to return a hash
because we're caching the stats and storing GeozoneStats objects would
need a lot more memory and we would get an error.

spec/lib/geozone_stats_spec.rb

@@ -0,0 +1,44 @@
+require "rails_helper"
+
+describe GeozoneStats do
+  let(:winterfell) { create(:geozone, name: "Winterfell") }
+  let(:riverlands) { create(:geozone, name: "Riverlands") }
+
+  describe "#name" do
+    let(:stats) { GeozoneStats.new(winterfell, []) }
+
+    it "returns the geozone name" do
+      expect(stats.name).to eq "Winterfell"
+    end
+  end
+
+  describe "#count" do
+    before do
+      2.times { create(:user, geozone: winterfell) }
+      1.times { create(:user, geozone: riverlands) }
+    end
+
+    let(:winterfell_stats) { GeozoneStats.new(winterfell, User.all) }
+    let(:riverlands_stats) { GeozoneStats.new(riverlands, User.all) }
+
+    it "counts participants from the geozone" do
+      expect(winterfell_stats.count).to eq 2
+      expect(riverlands_stats.count).to eq 1
+    end
+  end
+
+  describe "#percentage" do
+    before do
+      2.times { create(:user, geozone: winterfell) }
+      1.times { create(:user, geozone: riverlands) }
+    end
+
+    let(:winterfell_stats) { GeozoneStats.new(winterfell, User.all) }
+    let(:riverlands_stats) { GeozoneStats.new(riverlands, User.all) }
+
+    it "calculates percentage relative to the amount of participants" do
+      expect(winterfell_stats.percentage).to eq 66.667
+      expect(riverlands_stats.percentage).to eq 33.333
+    end
+  end
+end