Include number of downloads

The number of downloads can be checked through the GitHub API, e.g. using: https://githubstats0.firebaseapp.com/
For example username: nextcloud, repo: mail

Instead of using the "total downloads" which is just arbitrarily inflated, especially for apps with lots of releases, we could use the release with the largest number of downloads and say e.g. "12.000 users".

cc @jospoortvliet as talked about. cc @nextcloud/appstore