Fixed jump_to_id in problem solution.

Ticket: [TNL-729] (https://openedx.atlassian.net/browse/TNL-729)

If we include jump_to_id url in problem's solution like this ```<a href="/jump_to_id/c0f8d54964bc44a4a1deb8ecce561ecd">unit link</a>``` then it was not replaced with complete url of that unit which results a 404 page when we click on this link.
Called ```replace_jump_to_id_urls()``` on problem solution to complete the url.
Is it possible to test that the jump_to_id URL was replaced with the expected value? The fact that replace_jump_to_id_urls was called is not nearly as interesting to me as the fact that the output of get_answer is correct (it is an implementation detail).