Hi, a hopefully small feature request, please and thanks!

The "Name" and "Real name" are currently the only ways to search for someone.
At Wikimedia's installation, we've got custom fields for 3 alternate names: "IRC Nick", "LDAP User", and "Mediawiki User".
It would be awesome if we could search for (and autocomplete from) these alternate names.

E.g. https://phabricator.wikimedia.org/p/demon/ is currently going by the username "demon" and the real-name "Chad Horohoe". He lists his IRC name as "ostriches" and his mediawiki-username as "^demon" but neither of those names successfully autocomplete if I type them into the Subscribers field or site search.
E.g.2. ditto for https://phabricator.wikimedia.org/p/Rdicerb/ and her IRC name "rdaiccherlb".
E.g.3. ditto for https://phabricator.wikimedia.org/p/jrbs/ and his IRC/LDAP name "foks"

(downstream task at https://phabricator.wikimedia.org/T877)