Update acceptance tests to Selenium 3

The acceptance tests used the last Selenium 2 Docker container
available, which provides a rather old Firefox version (Firefox 47).
Nevertheless, despite some rendering issues, most things still worked as
expected due to the JavaScript files being built with support for older
browsers. However, now that support for Internet Explorer 11 and older
browsers will be dropped things could start to fail, so a newer browser
(and thus a newer Selenium version) should be used in the acceptance
tests.

Selenium has been standardized by the W3C, and the protocol to
communicate between the Selenium server and the browser has changed due
to that. Firefox >= 48 only supports the new W3C protocol, but the
Selenium driver for Mink does not support it yet.

The old protocol can still be used in recent Chromium/Chrome versions by
explicitly forcing it, so for the time being the acceptance tests will
need to be run on Chrome instead (although Firefox provides some
interesting features li...