Milestone M28 updates

Let's go through what we got done in Milestone M28 (Jan 13 - Jan 26).

• We have had migrations from exercise to challenge and then back to exercise over the last year in the page slugs of exercises for Bigbinary Academy. However, each such change had come with adding redirections to not let existing links break or SEO fail. When we reverted to exercise recently, although the github integration sync took care of adding redirections, older redirections don't make sense anymore. We wrote a script to cleanup the redirections and ensure new ones are in place, and we ran it.

• We found more edge cases in async code evaluation in Javascript and added a fix for one of the edge cases using our existing AST based solution.

• We also had issues with learners tampering with the fixed JS objects (like Array, Number etc) in code and evaluation breaking because of it. A relevant fix that prevents against it was added.