We call the `/u/search/users` URL when autocompleting users. It returns
user's name, username and avatar template, but not user ID.
We need it to return user IDs in order to display user status in certain situations.
I could add ID to FoundUserWithStatusSerializer, so it will be added only if
user status is enabled in site settings. But I feel that it's good to always return it,
it's not a lot of data comparing to what we already return, and it should be useful
in other scenarios.
