Change "Group#web_url" to return "/groups/twitter" rather than "/twitter".

Bring back the old behaviour which was changed by 6b90ccb9.
Fixes #23527.

app/models/group.rb

@@ -68,7 +68,7 @@ class Group < Namespace
   end
 
   def web_url
-    Gitlab::Routing.url_helpers.group_url(self)
+    Gitlab::Routing.url_helpers.group_canonical_url(self)
   end
 
   def human_name

config/routes/group.rb

@@ -12,7 +12,8 @@ constraints(GroupUrlConstrainer.new) do
   end
 end
 
-resources :groups, constraints: { id: /[a-zA-Z.0-9_\-]+(?<!\.atom)/ }  do
+scope constraints: { id: /[a-zA-Z.0-9_\-]+(?<!\.atom)/ } do
+  resources :groups, except: [:show] do
     member do
       get :issues
       get :merge_requests
@@ -31,4 +32,6 @@ resources :groups, constraints: { id: /[a-zA-Z.0-9_\-]+(?<!\.atom)/ }  do
 
       resources :labels, except: [:show], constraints: { id: /\d+/ }
     end
+  end
+  get 'groups/:id' => 'groups#show', as: :group_canonical
 end

spec/models/group_spec.rb

@@ -265,4 +265,10 @@ describe Group, models: true do
 
     members
   end
+
+  describe '#web_url' do
+    it 'returns the canonical URL' do
+      expect(group.web_url).to include("groups/#{group.name}")
+    end
+  end
 end