[UserMenteeApplication-2.0] Applicant code challenge submission

[JoshDevHub]

Application reviewers want to allow applicants to directly submit their code challenge on the app.

With the completion of #407, there will be a page for applicants to view their current application status. When they're on the coding_challenge_sent step, applicants should have a place on this page for submitting their code for the Agency's challenge.

This field could be part of the original application but as an optional field that gets filled in later.

Application reviewers should be able to see the submission on their end.

Blocked until completion of #407