Facia-tool OAuth 2.0

This changes `facia-tool` to use OAuth 2.0 from Google instead of OpenID.

It uses the library [`play-googleauth`](https://github.com/guardian/play-googleauth). There are updates that need to go through on that library such as https://github.com/guardian/play-googleauth/pull/7 and https://github.com/guardian/play-googleauth/pull/3. The new version should be either `0.1.5` or `0.1.6`, so ignore the version in `Frontend.scala` for now.

This also provides an `avatarUrl` for the logged in user.

#### What's Next
- Move `preview` onto a `Filter`
- Move `admin` onto `play-googleauth`
- Clean up all the old `Identity` stuff


#### Technical
This was originally meant to be a `Filter`. After trying to implement it as a `Filter`, I realised it could not be done as play (`2.2.x`) has no way of appending to the session, meaning that it was hard or impossible to update `lastSeenKey` in the `Session` without overwriting the `Session`. I changed to `Actions` and stuck with the `play-2.2.x` branch from `play-googleauth`. In the meantime we upgraded to play `2.3.x`. After this I decided to switch to `master` from `play-googleauth` which is based on play `2.3.x`. Then I realised that play `2.3.x` introduced `addingToSession` and `removingFromSession` meaning that the original method of a `Filter` would work. But this is going to stay as `Actions` for now.
Or maybe make it an option, and the AuthAction only kicks in if it is set.

Some kind of test to ensure it is never empty in PROD or CODE maybe...