Change actions for birthday event to include vgift purchase link.

This adds a link which only appears if vgifts are enabled in the config,
similar to the behavior of the contextual menu link.  However, this
link doesn't have the additional premission-checking logic of the
contextual menu link - but then again, neither do any of the other
items listed here.  In this context, where we are adding actions to
a message that the system sent, I think it's fine to just do the
config check.  Presumably some other part of the code is making sure
any undesirables aren't receiving these messages in the first place.

A side effect of the work here is to English-strip some code that was
not in the translation system before.  I also converted a couple of
unrelated BML::ml calls to LJ::Lang::ml, for the good of the order.