Hide Downloads from Metrics when there are no Downloadable galleys?

[StephDriver]

Question arising from working on #3911 which involved tidying up the empty Download lists when there were no galleys avaliable for download.

When an article has no downloadable Galleys, should the download metric be hidden from view?

Reasoning - having a metric with a zero just looks bad. And depending on screenwidth, it may not be onscreen at the same time as the 'there are no downloads' statement that now appears when there are none available.

Consideration - when an article used to be downloadable, but now isn't, i.e. has a non-zero metric for downloads, then it probably should continue to show that metric.

Proposed solution: have logic if galleys and if article.metrics.downloads i.e. downloads avaliable and non-zero for past downloads.