refs https://github.com/TryGhost/Team/issues/973
- added `@showCreate` option to `<GhInputWithSelect>` that enables the create option without having to pass a function in `@showCreateWhen`
- fixed autofocus not focusing input element in `<GhInputWithSelect>` when `autofocus="true"` isn't enough by itself
- updated `selectOrCreate` in `<GhInputWithSelect>` to trigger `onChange` if it's passed in rather than `onInput`
- swapped input in labs snippet input to `<GhInputWithSelect>`
- added actions for updating a snippet when an existing snippet is selected
- added action to editor control for saving an updated snippet
