Add the ability in the UI to export a list of all projects in a group and its subgroups to a csv

Background

The ability to obtain a list of all projects in a given group using the projects API currently exists, and one can get all the projects contained also in the subgroups using this API as well.

However, this is geared towards more technical users, and to transform the API output into a csv file requires scripting.

There are also ways to export certain items to csv already, as outlined here: https://docs.gitlab.com/ee/development/export_csv.html

Proposal

This issue proposes to add a functionality to also export a list of all projects to csv, by clicking a button in the UI.

This feature was suggested by a GitLab Ultimate customer in this Zendesk ticket (internal link only).