DEV: improve usability of subfolder specs

Previously people were not consistent about mocking which left internals in
a fragile state when running subfolder specs.

This introduces a simple helper `set_subfolder` which you can use to set
the subfolder for the spec. It takes care of proper configuration of subfolder
and teardown.

```
# usage
set_subfolder "/my_amazing_subfolder"
```

You should no longer stub base_uri or global_settings

spec/requests/application_controller_spec.rb

@@ -204,8 +204,7 @@ RSpec.describe ApplicationController do
       end
 
       it 'supports subfolder with permalinks' do
-        GlobalSetting.stubs(:relative_url_root).returns('/forum')
-        Discourse.stubs(:base_uri).returns("/forum")
+        set_subfolder "/forum"
 
         trashed_topic = create_post.topic
         trashed_topic.trash!

spec/requests/categories_controller_spec.rb

@@ -21,8 +21,7 @@ describe CategoriesController do
     end
 
     it 'web crawler view has correct urls for subfolder install' do
-      GlobalSetting.stubs(:relative_url_root).returns('/forum')
-      Discourse.stubs(:base_uri).returns("/forum")
+      set_subfolder "/forum"
       get '/categories', headers: { 'HTTP_USER_AGENT' => 'Googlebot' }
       html = Nokogiri::HTML(response.body)
       expect(html.css('body.crawler')).to be_present

spec/requests/list_controller_spec.rb

@@ -339,8 +339,7 @@ RSpec.describe ListController do
     end
 
     it 'renders links correctly with subfolder' do
-      GlobalSetting.stubs(:relative_url_root).returns('/forum')
-      Discourse.stubs(:base_uri).returns("/forum")
+      set_subfolder "/forum"
       post = Fabricate(:post, topic: topic, user: user)
       get "/latest.rss"
       expect(response.status).to eq(200)
@@ -458,8 +457,7 @@ RSpec.describe ListController do
         end
 
         it "renders RSS in subfolder correctly" do
-          GlobalSetting.stubs(:relative_url_root).returns('/forum')
-          Discourse.stubs(:base_uri).returns("/forum")
+          set_subfolder "/forum"
           get "/c/#{category.slug}.rss"
           expect(response.status).to eq(200)
           expect(response.body).to_not include("/forum/forum")

spec/requests/permalinks_controller_spec.rb

@@ -18,8 +18,7 @@ describe PermalinksController do
 
     it "should work for subfolder installs too" do
       permalink.update!(topic_id: topic.id)
-      GlobalSetting.stubs(:relative_url_root).returns('/forum')
-      Discourse.stubs(:base_uri).returns("/forum")
+      set_subfolder "/forum"
 
       get "/#{permalink.url}"
 

spec/requests/robots_txt_controller_spec.rb

@@ -49,7 +49,8 @@ RSpec.describe RobotsTxtController do
 
     context 'subfolder' do
       it 'prefixes the rules with the directory' do
-        Discourse.stubs(:base_uri).returns('/forum')
+        set_subfolder "/forum"
+
         get '/robots.txt'
         expect(response.body).to include("\nDisallow: /forum/admin")
       end

spec/requests/topics_controller_spec.rb

@@ -1861,7 +1861,8 @@ RSpec.describe TopicsController do
 
     describe 'clear_notifications' do
       it 'correctly clears notifications if specified via cookie' do
-        Discourse.stubs(:base_uri).returns("/eviltrout")
+        set_subfolder "/eviltrout"
+
         notification = Fabricate(:notification)
         sign_in(notification.user)
 
@@ -2146,8 +2147,7 @@ RSpec.describe TopicsController do
     end
 
     it 'renders rss of the topic correctly with subfolder' do
-      GlobalSetting.stubs(:relative_url_root).returns('/forum')
-      Discourse.stubs(:base_uri).returns("/forum")
+      set_subfolder "/forum"
       get "/t/foo/#{topic.id}.rss"
       expect(response.status).to eq(200)
       expect(response.body).to_not include("/forum/forum")

spec/requests/user_badges_controller_spec.rb

@@ -192,16 +192,9 @@ describe UserBadgesController do
     end
 
     describe 'with relative_url_root' do
-      before do
-        @orig_relative_url_root = ActionController::Base.config.relative_url_root
-        ActionController::Base.config.relative_url_root = "/discuss"
-      end
-
-      after do
-        ActionController::Base.config.relative_url_root = @orig_relative_url_root
-      end
-
       it 'grants badge when valid post/topic link is given in reason' do
+        set_subfolder "/discuss"
+
         admin = Fabricate(:admin)
         post = create_post