Search source filters users on username

It's very difficult to extend the search for users, because the SearchUsersSource component filters the users store based on the username. I was doing some work overriding the whole view of that component, that would however disable any extending by other extensions (eg masquerade wants to allow searching on custom fields, whereas user directory wants to add the link to the user directory).

A revamp of this component is required, I was considering an ItemList for results in addition to storing results in the store alone. But that's not a complete solution.

Start with the SearchUsersSource component and trace how it reads the users store and how extensions such as masquerade and user directory interact with it. Define the intended extensibility boundaries and a complete result-handling approach before changing the component; done means those extensions can add their behavior without overriding the whole view.

Written by the indexing model from the issue text.