Use Optional email address validator.

Change-Id: Id16ce13eac3bf4f359bd0d743cdd6041fc47bcaa
Depends-On: Ifc8ca757700db2dd19db89715e95f6c1df15e999
Reviewed-on: http://gerrit.dmdirc.com/3614
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Chris Smith <chris@dmdirc.com>