Give the textbox on edit template a better label

‘Message content’ is very much _our_ name for the thing. ‘Message’ is
more human-friendly.

app/main/forms.py

@@ -198,7 +198,7 @@ class SMSTemplateForm(Form):
         validators=[DataRequired(message="Can’t be empty")])
 
     template_content = TextAreaField(
-        u'Message content',
+        u'Message',
         validators=[
             DataRequired(message="Can’t be empty"),
             NoCommasInPlaceHolders()