replace dummy l10n messages with actual i18n keys

E.g. replace 'No_header' in {{l10n "No_header"}} with proper message key
'flow-header-empty'.  Note several special l10n keys remain because they need
special processing or parameters, such as 'Reply' (needs $author['gender']).

A follow-on patch will remove the dummy message handling from TemplateHelper.php.

Change-Id: I7979894f8b49e22419d60228a0a0281c0e773dc2