Support pure v5 tasks

[ThomasKranitsas]

We need to add support for a legacy property named pureV5Task (boolean).

If that property is set on a challenge, we will skip all legacy challenge checks and handle it purely on the v5 domain.

[ThomasKranitsas]

PRs:

• support pure v5 tasks #379
• stop back syncing (v4) for task topcoder-archive/legacy-challenge-processor#35
• support pure v5 tasks legacy-challenge-resource-processor#20

[rootelement]

Paid out via https://www.topcoder.com/challenges/24cb81b2-b403-465e-b2d2-13b10d29399d