Display virtual users (superadmin, Guest) with their user properties instead of their page name

$xwiki.getUserName / getPlainUserName and #displayUser have no special handling for the virtual users, which have no user page: they fall back to the raw page name, so superadmin is displayed as "superadmin" (e.g. in the page header and history of pages created at startup) and Guest as "XWikiGuest".

The user module already provides user properties for these virtual users (e.g. the superadmin configuration source with the first name "SuperAdmin"), but the display code doesn't use them.

Proposal

• Display the virtual users through their user properties (translated display name) in getUserName, getPlainUserName, #displayUser and the Live Data user displayer.
• Display them as plain text, without a link to a profile page that doesn't exist.

Context: phase 0 of the System user proposal (design: https://design.xwiki.org/xwiki/bin/view/Proposal/SystemUser). It's independent from it and can be done first.