Add email recipient username as a parameter to emailuser confirmation

Just as optional parameter, strictly following the bug summary, should be enough.
I sometimes open many tabs, forget who I emailed and have to check my email,
but unless someone thinks this common I don't think it's worth changing
the actual text of the confirmation adding more clutter.

Bug: 44668
Change-Id: Icf0ac18150847424728121e614849f87bee2e0e7