Add acceptance test steps to Drone

Each acceptance test feature is run in its own Drone step. The container
of the step runs the acceptance tests themselves, but they require two
additional Drone services. One service provides the Selenium server that
performs the web browser actions specified by the tests, and the other
service provides the Nextcloud server that the tests will be run
against (due to security concerns the acceptance tests themselves can
not create Docker containers for the Nextcloud server as done when
running them in a local system, as if Drone containers had access to
Docker a malicious pull request could be used to take over the Drone
server).

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>