Behat xpath issues cleanup

There are a few possible issues regarding behat tests and xpath that has not been properly checked during the 2.5 developement and we need to ensure they works as expected with all kind of arguments.

Copying from MDL-38613 description:

• When using text strings inside xpath strings they should not be escaped using \, they should use " or ' depending on whether the characters they contain
• XPath contains() should check class attributes wrapped in spaces
• XPath looking for contents to interact with should use equal comparators as much as possible to avoid things like clicking on "Users upload" instead of "Users" (that would be below it) when using I click on "Users"
• Look for steps definitions using hardcoded language string and replace them for get_string() calls