Feat: use react component for dialogs instead of window alerts

**Is your feature request related to a problem? Please describe.**
There are several places in the code that use `window.confirm/alert` for dialogs (one example is when uploading an existing media file).

I think we should should consider switching to a friendlier UI (maybe we can use the existing `redux-notifications` package we're using for status messages).

**Describe the solution you'd like**
Use a react component for dialogs instead of `window` dialogs.

**Describe alternatives you've considered**
We can write our own modals but I'm guessing there should be a library for that.

**Additional context**
Related to this https://github.com/netlify/netlify-cms/issues/2822