Improve UI and performance of branches overview page

What does this MR do?

Related issue: #211710 (closed)

The problem is that we're fetching all branches and then take 5 the most active and then iterate over the whole array until we get 5 branches with committer-date older than 3 months ago. If we display the stalest branches on the overview, we'll be able to fetch 5 the most recent and then fetch 5 the oldest via passing limit param to Gitaly. We perform 2 requests, but they are smaller and don't depend on the number of branches in the repository.

The changes are made according to the following reasoning: #211710 (comment 465275058)