registerWithAutocomplete on didInsertElement

In some cases, when transitioning to a view that uses ic-autocomplete, I was getting the error `Cannot read property 'focus' of null`, which refers to this line:
https://github.com/instructure/ic-autocomplete/blob/c14fac737a21072352f7d8d0ebd59af1df6c38fa/lib/autocomplete-option.js#L124

It seems like the component is trying to access its element before the element exists. Changing `willInsertElement` to `didInsertElement` fixes the bugs for me.