[#715] Checks if resource can be previewed and that changes the UX

Adds new helper function to check if a resource can be previewed and adds it to
the templates.
[#715] Adds more logic to the  helper
[#715] Tweaks to  method
Merge branch 'master' into 715-resource-preview-ux-changes
Merge branch 'master' into 715-resource-preview-ux-changes
[#715] Remove helper and do work in controller and minor tweaks
[#715] Fix can_be_previewed for embeds etc
[#715] Further cleanups
Merge pull request #715 from okfn/715-resource-preview-ux-changes

Change UX behaviour when displaying an external resource which can't be previewed