Implements spin-button

closes #3928
- Adds gh-spin-button component
- CSS for spinning button

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@PaulAdamDavis Can you check that CSS for me? The `spinner` doesn't spin currently (I re-used the CSS from .org for it). You can easily test it by setting `submitting=true` in the .hbs template.
I'd like to have the thing that gets rendered by default first - hence the unless.