Behat 3 upgrade

Upgrade to Behat 3

Changes:
- [x] Minor update on sentences
- [X] Update DemoContext to reflect changes of Behat Upgrade
- [x] Update ContentContext to reflect changes of Behat Upgrade

To do:
- [x] pass on that PR (https://github.com/ezsystems/ezpublish-community/pull/182)
> Tbh for me it looks much more usual to use *at* than *on*
Well, let's google it.
- "to click on": 670 000 000 results
- "to click at": "347 000 results

And yes, they should be strict *enough*. Not annoyingly strict, but strict
enough to match our domain. Fuzziness mostly makes sentences easier to
write, not easier to read. As long as we correctly document our corpus, we
shouldn't bother with too much flexibility, imho.