feat(input): interpolates form input name attrs for ngModel

@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.
Please don't add unobserve here, as we don't have it at similar places in the code neither and it confuses the reader (ask myself: "why here and not at the other places"...)