REFACTOR: Add `full_url` to `Group`

Lets stop writing the same code over and over again.

app/mailers/group_smtp_mailer.rb

@@ -77,7 +77,7 @@ class GroupSmtpMailer < ActionMailer::Base
     list = []
 
     post.topic.allowed_groups.each do |g|
-      list.push("[#{g.name_full_preferred}](#{Discourse.base_url}/groups/#{g.name})")
+      list.push("[#{g.name_full_preferred}](#{g.full_url})")
     end
 
     post.topic.allowed_users.each do |u|

app/mailers/user_notifications.rb

@@ -556,7 +556,7 @@ class UserNotifications < ActionMailer::Base
       participant_list = []
 
       post.topic.allowed_groups.each do |g|
-        participant_list.push "[#{g.name} (#{g.users.count})](#{Discourse.base_url}/groups/#{g.name})"
+        participant_list.push "[#{g.name} (#{g.users.count})](#{g.full_url})"
       end
 
       post.topic.allowed_users.each do |u|

app/models/group.rb

@@ -936,6 +936,10 @@ class Group < ActiveRecord::Base
     TopicAllowedGroup.where(group_id: self.id).joins(:topic).count
   end
 
+  def full_url
+    "#{Discourse.base_url}/g/#{UrlHelper.encode_component(self.name)}"
+  end
+
   protected
 
   def name_format_validator

spec/mailers/user_notifications_spec.rb

@@ -606,7 +606,7 @@ describe UserNotifications do
         notification_data_hash: notification.data_hash
       )
 
-      expect(mail.body).to include("[group1 (2)](http://test.localhost/groups/group1), [group2 (1)](http://test.localhost/groups/group2), [one](http://test.localhost/u/one), [two](http://test.localhost/u/two)")
+      expect(mail.body).to include("[group1 (2)](http://test.localhost/g/group1), [group2 (1)](http://test.localhost/g/group2), [one](http://test.localhost/u/one), [two](http://test.localhost/u/two)")
     end
 
     context "when SiteSetting.group_name_in_subject is true" do