Provide stable URLs for unit (as long as they exist)

weblate/trans/models.py

@@ -2065,7 +2065,7 @@ class Unit(models.Model):
         )
 
     def get_absolute_url(self):
-        return '%s?pos=%d&dir=stay' % (self.translation.get_translate_url(), self.position)
+        return '%s?checksum=%s' % (self.translation.get_translate_url(), self.checksum)
 
     def update_from_unit(self, unit, pos, force, template = None):
         '''

weblate/trans/views.py

@@ -1105,6 +1105,8 @@ def translate(request, project, subproject, lang):
             units = allunits.filter(position__lt = pos).order_by('-position')
         else:
             units = allunits.filter(position__gt = pos)
+    elif 'checksum' in request.GET:
+        allunits = obj.unit_set.filter(checksum = request.GET['checksum'])
     else:
         allunits = obj.unit_set.filter_type(rqtype, obj)
         # What unit set is about to show