Force Nameprepping of JID domain parts

The IDN.toAscii()/IDN.toUnicode() family only namepreps the original
domain passed to it if it contained non-ASCII characters. This means
that for all-ASCII domains, no canonicalization is performed, which
leads to issues like case-sensitivity. This workaround explicitly
namepreps domain parts before calling IDN.toAscii() on them, in order to
get a canonicalized representation (most notably, case invariance). A
basic DB migration is also included.