FEATURE: Add order logic to admin users controller

Added order and direction parameters for sorting admin user pages. This
commit only includes backend api changes.

https://meta.discourse.org/t/make-admin-users-list-sortable-suggestion/47649

Now you can pass in `order` and `asc` parameters to the
`/admin/users/list/<query>.json` endpoint.

Example:

`/admin/users/list/active.json?&order=post_count` which defaults to desc

and

`/admin/users/list/active.json?order=post_count&asc=true`

spec/components/admin_user_index_query_spec.rb

@@ -23,9 +23,24 @@ describe AdminUserIndexQuery do
     end
 
     it "allows custom ordering" do
-      query = ::AdminUserIndexQuery.new({ order: "trust_level DESC" })
+      query = ::AdminUserIndexQuery.new({ order: "trust_level" })
       expect(query.find_users_query.to_sql).to match("trust_level DESC")
     end
+    
+    it "allows custom ordering asc" do
+      query = ::AdminUserIndexQuery.new({ order: "trust_level", asc: true })
+      expect(query.find_users_query.to_sql).to match("trust_level ASC" )
+    end
+
+    it "allows custom ordering for stats wtih default direction" do
+      query = ::AdminUserIndexQuery.new({ order: "topics_entered" })
+      expect(query.find_users_query.to_sql).to match("topics_entered DESC")
+    end
+
+    it "allows custom ordering and direction for stats" do
+      query = ::AdminUserIndexQuery.new({ order: "topics_entered", asc: true })
+      expect(query.find_users_query.to_sql).to match("topics_entered ASC")
+    end
   end
 
   describe "no users with trust level" do
@@ -70,6 +85,28 @@ describe AdminUserIndexQuery do
 
   end
 
+  describe "correct order with nil values" do
+    before(:each) do
+      Fabricate(:user, email: "test2@example.com", last_emailed_at: 1.hour.ago)
+    end
+
+    it "shows nil values first with asc" do
+      users = ::AdminUserIndexQuery.new({ order: "last_emailed_at", asc: true }).find_users
+
+      expect(users.count).to eq(2)
+      expect(users.first.username).to eq("system")
+      expect(users.first.last_emailed_at).to eq(nil)
+    end
+
+    it "shows nil values last with desc" do
+      users = ::AdminUserIndexQuery.new({ order: "last_emailed_at"}).find_users
+
+      expect(users.count).to eq(2)
+      expect(users.first.last_emailed_at).to_not eq(nil)
+    end
+
+  end
+
   describe "with an admin user" do
 
     let!(:user) { Fabricate(:user, admin: true) }