[learn] save challenges locally so there is no need for internet

[Sem Bauke]

This is something up for discussion. We might need to do something similar to news.

[Niraj Nandish]

I'm reopening this as this includes even allowing the users to be able to submit the challenges offline and track their progress. Once they connect to internet, we should come up with a logic of merging the user object and updating it in the DB.