UX: Lack of user waiting affordance when first-use downloading from Unpkg

Hi.

There's a series of UX / UI issues I'm encountering while onboarding clients to using Netlify CMS and I thought I'd open issues for each of the key items. My apologies if these are brief, but I'm on a deadline and I don't want to nerd-snipe myself trying to speculatively produce solutions or mocks etc.

First up - on the first use of Netlify CMS in a browser, it looks to me like hitting 'Log in with Netlify Identity' causes a large-ish download of assets from Unpkgd (?). Unless the user knows to look at their browser's network resource indicator bar, the button appears to have done nothing.

I'd propose adding a spinner on the first click of the button, so that there's an indication that there has indeed been a successful user action taken place here.