Add a hint about how to use placeholders

Since placeholders (almost) work now, it’s worth telling people what the syntax
is.

This commit also removes the ‘template type’ picker, since you can only create
SMS templates at the moment. This will be revisited when we start looking at how
you add an email template.

app/main/forms.py

@@ -188,7 +188,6 @@ class TemplateForm(Form):
     name = StringField(
         u'Template name',
         validators=[DataRequired(message="Template name cannot be empty")])
-    template_type = RadioField(u'Template type', choices=[('sms', 'SMS')])
 
     template_content = TextAreaField(
         u'Message',