Download options for all formats when some formats unavailable

**Description**

When you click download button several options appear such as PDF ePUB kindle etc. These are still available when the formats aren't there resulting in links which indicate a failure to generate the content.

**Evidence / Screenshot (if possible)**

The kindle link on this book:
http://www.archive.org/stream/birdbookillustra00reedrich#page/n4/mode/2up

**Expectation**

Only display the available formats. Ideally, if there is only one format available, only link to this download.

**Context**

Occurs in all cases.

**Proposal & Constraints**

An if statement which checks if available downloads is one. If so directly link to this. Else open the popup and only display available downloads. Rename the button to display 'Download' or a symbol rather than 'PDF/ePUB'.

**Stakeholders**