Fix the majority of verification race conditions

Data races can probably still occur in the following cases:
- If Discord is still adding the role when we fetch the user for the
  second time
- If we don't all verify, for example if someone does a kick and someone
  else does a verify

These are substantially less likely than the common case, which has
happened several times now

This was ported over from TransGG/theo#5

src/verification/managers/verifyUser.js

@@ -42,6 +42,11 @@ async function verifyUser(ticket, verifier, resolve, reject) {
         return;
     }
 
+    if (applicant.roles.cache.has(config.roles.verified)) {
+        await reject('User has already been verified');
+        return;
+    }
+
     // verify user
     await verify(applicant);