(bug 31739) Made Block code support ipb_by = 0 convention with for foreign users...

(bug 31739) Made Block code support ipb_by = 0 convention with for foreign users again, as it did pre 1.18. The byText param has been restored in the Block() constructor as CentralAuth still uses it. Some callers have been updated to reflect the fact that getBlocker() may be a local User or a string username.

Ideally, we might have a ForeignUser class and a generic User interface...but this will do for now.