Store total downloads in each plugin's manifest

It would be good to centrally record the total downloads of a plugin,
For example so that it can be shown in the plugin store (app & website)

This is potentially a sub issue of #284 as it would be good to cache this during any sorting by it

The existing github api for releases does provide this per release so we would need to loop through and sum them
`https://api.github.com/repos/{username}/{repo_name}/releases`

Trace how plugin manifests are generated and consumed in this repository, then inspect the GitHub releases API response for the per-release download counts. Done means each plugin manifest centrally records the summed total downloads so the plugin store can display it; check whether the proposed sorting cache in issue #284 affects the scope.

Written by the indexing model from the issue text.