Audit can not retrieve users anymore

[Zabuzard]

The recent merge #472 messed up the ability for /audit to retrieve the actual users who used the actions against the target.

See the following example:

Reason

The bug comes from exchanging the previously correct

jda.retrieveUserById(action.authorId())

by the wrong

jda.getUserById(action.authorId());

Latter only checks the JDA cache instead of asking Discord. The cache is rarely reliable (only for the list of guilds).

Changing this back is fairly simple, but requires to make the code properly async using RestAction.

[Zabuzard]

FYI @Taz03

(also, please dont forget to test on the test server before deploying, thanks)