Improve 'auto fsck' admin emails

app/mailers/repository_check_mailer.rb

@@ -8,7 +8,7 @@ class RepositoryCheckMailer < BaseMailer
 
     mail(
       to: User.admins.pluck(:email),
-      subject: @message
+      subject: "GitLab Admin | #{@message}"
     )
   end
 end

app/views/repository_check_mailer/notify.html.haml

@@ -3,3 +3,6 @@
 
 %p
   = link_to "See the affected projects in the GitLab admin panel", admin_namespaces_projects_url(last_repository_check_failed: 1)
+
+%p
+  You are receiving this message because you are a GitLab administrator for #{Gitlab.config.gitlab.url}.

app/views/repository_check_mailer/notify.text.haml

@@ -1,3 +1,6 @@
 #{@message}.
 \
 View details: #{admin_namespaces_projects_url(last_repository_check_failed: 1)}
+
+You are receiving this message because you are a GitLab administrator
+for #{Gitlab.config.gitlab.url}.

spec/mailers/repository_check_mailer_spec.rb

@@ -15,7 +15,7 @@ describe RepositoryCheckMailer do
     it 'mentions the number of failed checks' do
       mail = described_class.notify(3)
 
-      expect(mail).to have_subject '3 projects failed their last repository check'
+      expect(mail).to have_subject 'GitLab Admin | 3 projects failed their last repository check'
     end
   end
 end