Try keep bots off of our beacon...

1. Apply a `robots.txt` to the beacon server
2. Put `a rel="no-follow"` on the `<img>` tag in the html
Add the ability to log Beacon requests details.

Behind a switch as there are a lot of requests.
Merge branch 'master' of github.com:guardian/frontend into no-follow

Conflicts:
	common/app/conf/switches.scala
Merge branch 'master' of github.com:guardian/frontend into no-follow

Conflicts:
	common/app/conf/switches.scala
Merge pull request #2735 from guardian/no-follow

Try keep bots off of our beacon...