Unify handling of *-message(s) settings in HTMLForm

Unify handling of *-message(s) settings in HTMLForm

*-message(s) settings were documented as message key strings or arrays
of message key strings, but some actually accepted [key, params...]
arrays as well. They did not accept Message objects, which would be
the cleanest and most flexible method of message passing.

The patch adds a new method to process these settings (which accepts
a messages key, a [key, params...] array or a Message object), and
makes all *-message(s) usage call that.

Change-Id: Ida647973a58bea83fdbd53335e63b5a8615c16e4