[REF] MessageTemplate form code level improvements

[REF] MessageTemplate form code level improvements

I'm looking to cleanup this form to use the apiv4 (I'd rather got the extra step & switch it to an afform but
that seems like too big a leap).

This switches the loading to use apiv4. Note that
1) I decided that it doesn't make sense to setCheckPermissions = FALSE - I think the form should not
be availble to non-permissioned users (& perhaps a hook might like to play a role here).

2) I removed the inheritence from the parent  which seemed to do 3 things
 - added admin.css - none of the classes seemed to apply
 - added iconpicker - didn't seem to apply
 - loaded the defaults - which this change does on the form more succinctly