Fix validation of strings with leading and trailing newlines (#3003)

We use CharFields for validating original strings and translations when translations are being saved.

By default, CharField strips the value of the leading and trailing whitespace:
https://docs.djangoproject.com/en/4.2/ref/forms/fields/#django.forms.CharField

Until now, we've been only bypassing this default for translations. From now on, we also do that for original strings.

pontoon/translations/forms.py

@@ -51,5 +51,8 @@ class CreateTranslationForm(forms.Form):
             return None
         return self.cleaned_data["plural_form"]
 
+    def clean_original(self):
+        return self.data.get("original", "")
+
     def clean_translation(self):
         return self.data.get("translation", "")