Remove embargo, fix several issues

* Removes the embargo functionality from the ui
* Adds a file download button to the publication detail page
* Adds a back button to the publication detail page which takes the user back to the home page

TL;DR: Fixes https://github.com/CUL-DigitalServices/avocet-ui/issues/266, https://github.com/CUL-DigitalServices/avocet-ui/issues/261 and https://github.com/CUL-DigitalServices/avocet-ui/issues/257

This can't be merged before there is a Hilary counterpart to this PR.