Legacy UI: Previous button should use prev_url if it in first
position. In studio UI, it should always use prev_url. To make this
possible we checking length of contents which is always zero for studio
and more than 0 for legacy UI.
- Moving xmodule folder to root as we're dissolving sub-projects of common folder in edx-platform
- More info: https://openedx.atlassian.net/browse/BOM-2579
- -e common/lib/xmodule has been removed from the requirements as xmodule has itself become the part of edx-platform and not being installed through requirements
- The test files common/lib/xmodule/test_files/ have been removed as they are not being used anymore
