help ensure key uniqueness for token field items

[kdoh]

A stab at resolving #7043.

TokenField was using the value as a key for the list items in the token field list, which in the case of the pulse recipient list was potentially an entire user object. I think that was causing React to swallow the new items during the DOM update given it was logging this message.

Encountered two children with the same key, [object Object]. Child keys must be unique; when two children share a key, only the first child will be used. - Pretty much sums up the behavior we saw.

This is a dumb fix to confirm what needs to be changed as index is somewhat frowned upon as a key (which is why I'm guessing this happened in the first place). There are a couple options as I see it:

1. Use the v + index solution I did here (probably a bad idea, but hey it works)
2. Allow the user of the <TokenField /> component to provide a custom key prop and if it's not specified fall back to index.
3. Something smarter @tlrobinson or someone else comes up with.

[tlrobinson]

Using the index should be ok for this purpose.