Download sampletype template is shown and available if can_download? is false

[stuzart]

Previously the Template could always be downloaded if the sample type was visible, but since the introduction of permissions can_download? has been added as an option. As discovered with #2076 the download button is shown, but only if there is permissions, but the download link on the sample type view page is always shown and accessible if visible.

should also make sure and add tests that the action is blocked if can_download? is false.

There is a question of whether this separate permission is needed, as downloading the template doesn't reveal anything not already known.