Link directly to placeholders tab

This MR updates the `PendingReassignmentAlertPresenter` banner to link
directly to the placeholders tab.

This is to take the user directly to the information the banner informs
them of, and in most cases saves them an extra click.

Related issue: https://gitlab.com/gitlab-org/gitlab/-/issues/495533

app/presenters/import/pending_reassignment_alert_presenter.rb

@@ -55,7 +55,7 @@ def group_links
       groups_awaiting_placeholder_assignment.collect do |namespace|
         placeholders << "%{group_#{namespace.id}_link_start}#{namespace.name}%{group_#{namespace.id}_link_end}"
         tag_pairs << tag_pair(
-          tag.a(href: group_group_members_path(namespace)),
+          tag.a(href: group_group_members_path(namespace, tab: 'placeholders')),
           :"group_#{namespace.id}_link_start",
           :"group_#{namespace.id}_link_end"
         )

spec/presenters/import/pending_reassignment_alert_presenter_spec.rb

@@ -57,7 +57,7 @@
 
     it 'presents the import values' do
       expect(presenter.show_alert?).to eq(true)
-      expect(presenter.group_links).to eq("<a href=\"/groups/blink/-/group_members\">blink</a>")
+      expect(presenter.group_links).to eq("<a href=\"/groups/blink/-/group_members?tab=placeholders\">blink</a>")
       expect(presenter.groups_awaiting_placeholder_assignment).to match_array(namespaces)
       expect(presenter.group_names).to eq('blink')
       expect(presenter.source_hostname).to eq('gitlab.example')
@@ -86,9 +86,9 @@
     it 'presents the import values' do
       expect(presenter.show_alert?).to eq(true)
       expect(presenter.group_links).to eq(
-        "<a href=\"/groups/blink/-/group_members\">blink</a>, " \
-          "<a href=\"/groups/marquee/-/group_members\">marquee</a>, " \
-          "and <a href=\"/groups/details/-/group_members\">details</a>"
+        "<a href=\"/groups/blink/-/group_members?tab=placeholders\">blink</a>, " \
+          "<a href=\"/groups/marquee/-/group_members?tab=placeholders\">marquee</a>, " \
+          "and <a href=\"/groups/details/-/group_members?tab=placeholders\">details</a>"
       )
       expect(presenter.groups_awaiting_placeholder_assignment).to match_array(namespaces)
       expect(presenter.group_names).to eq('blink, marquee, and details')