fixes https://github.com/TryGhost/Team/issues/2433
- Moved all outbound link tagging code to separate OutboundLinkTagger
- Because a site can easily enable/disable this feature, we don't store
the ?refs in the HTML but add them on the fly for now in the Content
API.
closes https://github.com/TryGhost/Team/issues/1916
closes https://github.com/TryGhost/Team/issues/1917
- Added database storage for link redirects and click events via repositories (hides away database layer) defined in the wrapper services
- Added LinkClickRepository to store click events to database
- Added LinkRedirectRepository to store link redirects to database
- Added PostLinkRepository to link LinkRedirects with posts
- Renamed link-replacement package to link-replacer, and made it dependency less (it only replaces links now, doesn't do anything else)
- The link-tracking service has a new `addTrackingToUrl` which returns a new URL that includes tracking. The new `addRedirectToUrl` method does the same but without tracking for now.
- MEGA service now uses the link-replacer to replace links in the emails using a combination of different services (member attribution + link-tracking service)
