User search screen should order prefix matches first

At present, user search results are shown with all matches in alphabetical order. This doesn't match what the user is trying to do, which is usually to put in the start of someone's name. It looks especially "broken" when the user first start typing, as it's unclear why these particular names are being shown, or why the name list didn't seem to change (or change much) when the first letter was typed.

We should change the ordering to show prefix matches with any part of the name above other results, the way we currently do in the right sidebar of the web app. It should be fine to just use <space> as the separator, or match whatever implementation we have in the web app.