Add validation for whiteboard messages

Issue #159
Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/models/whiteboard.py

@@ -21,7 +21,8 @@
 """Whiteboard model."""
 
 from django.db import models
-from django.utils.translation import ugettext_lazy
+from django.utils.translation import ugettext_lazy, ugettext as _
+from django.core.exceptions import ValidationError
 from weblate.lang.models import Language
 
 
@@ -41,3 +42,12 @@ class WhiteboardMessage(models.Model):
 
     def __unicode__(self):
         return self.message
+
+    def clean(self):
+        if self.project and self.subproject:
+            if self.subproject.project == self.project:
+                self.project = None
+            else:
+                raise ValidationError(
+                    _('Do not specify both component and project!')
+                )