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.
@cahrens We can't achieve this in this test because ```replace_jump_to_id_urls``` function is in common/djangoapps and we can't import it in xmodule lib thats why i mocked this function and just verified that it is called. If you want this functionality then i can write a bok-choy test for it.