MDL-45192 assign: added behat script & new definitions to handle alert dialog

The behat script is to test the batch action to lock and unlock submissions.
The new definitions are to prevent the "unexpected alert open" error in Selenium.
MDL-45192 behat: Replacing specific-selector steps by a generic one

Having a I click on we can use all our selectors rather
than having multiple steps for the same purpose. This
step allow us to manage JS native dialogues without
having to fail a scenario.

Credit for Mohamed Alsharaf.