MDL-44326 behat: Removing a switch theme

This scenario is using afterburner theme but
there is nothing special that makes us use it,
removing it as it will fail if any 3rd party
using ->theme extends bootstrapbase.
MDL-44326 behat: Refactoring drag&drop steps

Also added a new step to check the current theme as
themes can be specified in config.php and all tests
expecting selectors not available in the selected theme
will fail.
MDL-44326 behat: Add DD tests to move activities feature
Moodle MDL-41551 correct block drag and drop on /my page

_ add the layout/my.php layout file to bootestrapbase and clean with the block region generating PHP output method for the "content" region blocks
_ list the "content" region in the bootstrapbase "mydashboard" layout regions array
_ add the "region-content" strings to the bootstrapbase and clean lang files
_ extend the /my page related JavaScript in blocks/js/manager.js to add the block region on pages where the "content" block region is not present