Commit 0cdaaa1d

 introduced a lot of calls to display_default_name.
Unfortunately this bypasses the user record caching that happens
inside display_name, so it produced a lot of new queries, most notably
on profile pages, where a single user's name gets displayed several
times.

This change moves the caching of user records into another function,
so it can be shared by display_name, display_default_name, and
profile_icon_url.

The search_user function is also modified to fetch the admin status
and profileicon id, to stop profile_icon_url producing more queries.

Change-Id: Id1993decf164e7d0563a7d277444402fc6350b3a
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>