fix: add return after making server response

[halfwhole]

Problem

In two places in the codebase, the server does not return immediately after making a response, leading to res.send() being called twice, which causes an unhandled promise rejection. This is still okay for node <14 because it just gives a warning, but not from node 15 onwards (see node 15 release docs) as it throws an error and terminates the entire node process, which is very scary.

Example error message:

Solution

• In UserController: simply add return
• In LinkAuditController: had to invert the logic a little to do the same functionality without calling res.send() twice

[gweiying]

lgtm