Tweak styles of document list filters and results

* [x] Code review
* [x] Product review

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* Use smart quotes in titles
* Update placeholder copy to match content style guide
* Use a Bootstrap button for filter form
* Pluralise document counts correctly
* Make more use of ported admin template styles for no content states and table headers

Before

![screen shot 2014-08-20 at 11 21 52](https://cloud.githubusercontent.com/assets/319055/3979403/00b0917e-2854-11e4-8354-64bd7cdb4a4e.png)
![screen shot 2014-08-20 at 11 22 26](https://cloud.githubusercontent.com/assets/319055/3979404/00d4f348-2854-11e4-90d2-ed331c355b3e.png)

After

![screen shot 2014-08-20 at 11 15 49](https://cloud.githubusercontent.com/assets/319055/3979369/8bd69ac4-2853-11e4-96b8-c970450bd840.png)

![screen shot 2014-08-20 at 11 16 09](https://cloud.githubusercontent.com/assets/319055/3979370/8bd6ed58-2853-11e4-916e-e4b8cc9d65b8.png)

![screen shot 2014-08-20 at 11 19 34](https://cloud.githubusercontent.com/assets/319055/3979368/8bd02df6-2853-11e4-8038-310996cf3735.png)