Add autofocus to Identity form helpers
autofocus is a boolean type, not a string

Add support for arbitrary attribute types for Input template
representations. This allows us to more tersely include attributes
that influence merely by their presence or absence (e.g. autofocus,
rather than autocomplete="on") using a boolean.
Merge pull request #2083 from guardian/identity-forms-autofocus

Add autofocus to Identity form helpers