FEATURE: optional global invite_code for account registration

On some sites when bootstrapping communities it is helpful to bootstrap
with a "light weight" invite code.

Use the site setting `invite_code` to set a global invite code.

In this case the administrator can share the code with
a community which is very easy to remember and then anyone who has
that code can easily register accounts.

People without the invite code are not allowed account registration.

Global invite codes are less secure than indevidual codes, in that they
tend to leak in the community however in some cases when starting a brand
new community the security guarantees of invites are not needed.

spec/requests/users_controller_spec.rb

@@ -593,8 +593,8 @@ describe UsersController do
         email: @user.email }
     end
 
-    def post_user
-      post "/u.json", params: post_user_params
+    def post_user(extra_params = {})
+      post "/u.json", params: post_user_params.merge(extra_params)
     end
 
     context 'when email params is missing' do
@@ -616,17 +616,27 @@ describe UsersController do
         expect(User.find_by(username: @user.username).locale).to eq('fr')
       end
 
+      it 'requires invite code when specified' do
+        expect(SiteSetting.require_invite_code).to eq(false)
+        SiteSetting.invite_code = "abc"
+        expect(SiteSetting.require_invite_code).to eq(true)
+
+        post_user(invite_code: "abcd")
+        expect(response.status).to eq(200)
+        json = JSON.parse(response.body)
+        expect(json["success"]).to eq(false)
+
+        post_user(invite_code: "abc")
+        expect(response.status).to eq(200)
+        json = JSON.parse(response.body)
+        expect(json["success"]).to eq(true)
+
+      end
+
       context "when timezone is provided as a guess on signup" do
-        let(:post_user_params) do
-          { name: @user.name,
-            username: @user.username,
-            password: "strongpassword",
-            email: @user.email,
-            timezone: "Australia/Brisbane" }
-        end
 
         it "sets the timezone" do
-          post_user
+          post_user(timezone: "Australia/Brisbane")
           expect(response.status).to eq(200)
           expect(User.find_by(username: @user.username).user_option.timezone).to eq("Australia/Brisbane")
         end
@@ -1440,7 +1450,7 @@ describe UsersController do
       inviter = Fabricate(:user, trust_level: 2)
       sign_in(inviter)
       invitee = Fabricate(:user)
-      invite = Fabricate(:invite, invited_by: inviter, user: invitee)
+      _invite = Fabricate(:invite, invited_by: inviter, user: invitee)
       get "/u/#{user.username}/invited_count.json"
       expect(response.status).to eq(200)