Separate client side build from SBT.

Try this without whitespace in diffs by adding [?w=1 to the URL.](https://github.com/guardian/frontend/pull/1897/files?w=1)

Farewell to the `AssetsPlugin`. Our primary aim here is to remove the asset hashing nonsense from the SBT build and as it happens everything basically collapses. Yes, asset hashing was the rotten core of the SBT build and most of our custom SBT build configuration is now removed.

The Grunt build is similarly improved in that it is solely responsible for building assets now. The clientside of the table owns this file btw and masters of their own destiny. So long suckers! :)

Some things to note:

* The grunt asset hashing plugin has a new scheme and will cache bust everything. Sorry.
* Added `grunt-watch` so new assets are compiled in the background.
* @adamnfish Given your pull request removes the secure properties, I've not bothered to port them to the assets plugin replacement here. Sorry, you were warned. ;)
* Admin integration tests have been disabled until Facia get them working in dev again. They were broken by separating the fronts-tool from admin.
* There will be a follow up PR to clean up some of the details, i.e. move the asset source files under static from common, maybe generate the sprite to `static/target`, etc.
* Requires some simultaneous teamcity build config changes. Yes, chaos on merge. I'll watch it.
* I'll email instructions on new development practice post merge. Basically a repetition of the Running section from new `README.md`.

@phamann @gklopper Reviews please. You have until Friday.