Display pretests differently in UI

[Xyene]

Split out from https://github.com/DMOJ/judge-server/issues/1068:

Write site patch to display pretest cases as pretests even when not in a pretested contest.

We probably want to add an indicator in the judge-site protocol for whether a case is a pretest or not. We can infer today from total = 0, but we want to move towards a world where pretests can have nonzero points but not count towards a submission's total. This requires separate tagging.

We should:

• Display pretests as "Pretest test case #x" instead of "Test case #x", and "Pretest batch #x" instead of "Batch #x"
• The counter for pretests should be different from the counter for regular cases, i.e. we should be able to display "Pretest test case #‍1" followed by "Test case #‍1".

Later, we will smash all init.ymls to rewrite 0-point testcases as pretests with appropriate pretest dependencies for following cases.