Improve Backlog by showing all MRs in the opened state by week

[alpgarcia]

This was requested by @rpaik at https://gitlab.com/Bitergia/c/gitlab/support/issues/31

We should include the outcomes of that task in the existing Backlog dashboard for MRs or create a new specific dashboard to show it if needed. It is part of the task to make this decision based on having a clear use case and not a bunch of visualizations with no clear use case relating them. That use case should be documented as well.

Closing condition: visualization(s) are part of our dashboards, documentation is updated accordingly and optionally, new tasks are created to expand this to other similar sources (GitLab Issues, GitHub, Gerrit, Jira, one ticket per source to allow to evaluate if this make sense there and how to implement it). Don't create any task before completing this to make sure we agree a clear scope before starting adding things that later may end being misleading due to differences among dashboards that should look similar.