Add link tracking

[muffinresearch]

Fixes mozilla/addons#9805

[kumar303]

Looks like you'll need to rebase this on master to resolve the conflict and also to check that the new lint rules aren't nagging you about anything.

[mstriemer]

I feel like this should be in core/tracking. It might make sense to specify that this is a tracking action as well by naming it trackingAction or getTrackingAction since "action" is a term used in redux as well.

[mstriemer]

Actually, if this were in core/tracking it could probably be part of tracking.sendEvent:

export function sendEvent({ category, action, label, type, value } = {}) {
  if (type !== undefined) {
    action = getAction(type);  // This won't pass eslint but you know.
  }
  // ...
}

[mstriemer]

I can see a case for keeping getAction out of tracking.sendEvent since it is likely specific to extensions and themes but changing it's module might still be a good idea if that's the case.

r+wc