PagesDomain: improve breadcrumbs

app/views/projects/pages_domains/edit.html.haml

+- add_to_breadcrumbs "Pages", project_pages_path(@project)
+- breadcrumb_title @domain.domain
 - page_title @domain.domain
 %h3.page_title
   = @domain.domain

app/views/projects/pages_domains/new.html.haml

+- add_to_breadcrumbs "Pages", project_pages_path(@project)
 - page_title 'New Pages Domain'
 %h3.page_title
   New Pages Domain

app/views/projects/pages_domains/show.html.haml

+- add_to_breadcrumbs "Pages", project_pages_path(@project)
+- breadcrumb_title @domain.domain
 - page_title "#{@domain.domain}", 'Pages Domains'
+
 - verification_enabled = Gitlab::CurrentSettings.pages_domain_verification_enabled?
 - if verification_enabled && @domain.unverified?
   %p.alert.alert-warning