Introduce Prev/Next pagination to branches

Gitaly provides page-token pagination for branches instead of
offset pagination.

That means that we can't easily access a particular page of
the branches list without fetching all branches from Gitaly
and iterate them manually.

However, page-token pagination allows us to retrieve next bunch
of branches. We can use it in order to implement prev-next
pagination which should work more performant.

app/views/projects/branches/index.html.haml

@@ -60,7 +60,10 @@
   %ul.content-list.all-branches
     - @branches.each do |branch|
       = render "projects/branches/branch", branch: branch, merged: @merged_branch_names.include?(branch.name), commit_status: @branch_pipeline_statuses[branch.name], show_commit_status: @branch_pipeline_statuses.any?
-  = paginate @branches, theme: 'gitlab'
+  - if Feature.enabled?(:branches_pagination_without_count, @project, default_enabled: true)
+    = paginate_without_count @branches
+  - else
+    = paginate @branches, theme: 'gitlab'
 - else
   .nothing-here-block
     = s_('Branches|No branches to show')

config/feature_flags/development/branches_pagination_without_count.yml

+---
+name: branches_pagination_without_count
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/50759
+rollout_issue_url:
+milestone: '13.9'
+type: development
+group: group::source code
+default_enabled: true