Display the number of known sites that have the plugin installed

It is known that the download stats are not very valuable. There is a constant demand for getting know how many sites out there have the particular plugin actually installed.

At the moment, the only reliable source for such stats are requests for the available updates notifications. We have been gathering anonymous data about plugins usage for some time now and it is time to display them at the plugins directory page.

The plan is:

1. Extend the plugin Stats page so that it shows the number of known installations in time
2. Display the latest known figure next to the number of favourites and number of downloads

To make that happen, some co-operation with the download.moodle.org site will be needed which I am going to elaborate in subtasks.