Flock: added Advisory subscription support

see https://github.com/GigaOM/legacy-pro/issues/4525
For better centering and to more closely match the guide, I'd add

```sass
.go-radio:checked {
	+ span:before {
		top: 0px;
	}
	+span:after {
		top: -1px;
	}
}
```

I looked at this and tried to (unsuccessfully) figure out what was different between the two...we might want to make this a research-wide override...