Form input 'name' attribute should be interpolated

@frankhuster / @niargh has brought up an issue with using the `$index` of a repeated element for the name attribute of a form input.

I don't know how common of a use-case it is, but it seems not especially harmful to do a one-time interpolation in post-link.

It shouldn't be a breaking change unless people are using interpolation markers in their names.