Show branch preview banner on every (non-master) page

[andylolz]

E.g. it’s not currently shown on http://org-id.guide/results .

I think it’s fine to use cookies for branch previewing – it’s probably the correct way to do it. But when I’m previewing a branch and I want to test how the new codelist appears in the results, it’s not obvious that the cookie is still set because the preview banner disappears. It would be good if the banner were there on every page.

Related to this, it would probably be preferable to use middleware to inject the branch variable into templates, rather than having to handle it for each view.