Extract method to get a user segment name

We're going to add geozones as user segments, so it's handy to have the
method in the UserSegments class.

We're also changing the `user_segment_emails` parameter name for
consistency and simplicity.

spec/lib/user_segments_spec.rb

@@ -5,6 +5,22 @@ describe UserSegments do
   let(:user2) { create(:user) }
   let(:user3) { create(:user) }
 
+  describe ".segment_name" do
+    it "returns a readable name of the segment" do
+      expect(UserSegments.segment_name("all_users")).to eq "All users"
+      expect(UserSegments.segment_name("administrators")).to eq "Administrators"
+      expect(UserSegments.segment_name("proposal_authors")).to eq "Proposal authors"
+    end
+
+    it "accepts symbols as parameters" do
+      expect(UserSegments.segment_name(:all_users)).to eq "All users"
+    end
+
+    it "returns nil for invalid segments" do
+      expect(UserSegments.segment_name("invalid")).to be nil
+    end
+  end
+
   describe ".all_users" do
     it "returns all active users enabled" do
       active_user = create(:user)