Add a license preference field

This feature is absolutely necessary. Now a user can simply define
their default license and quickly go through a form, as opposed to
stopping to click on the select and choosing the same option over
and over again.

Also added DB migration for the field, so that's working now, too.

Rebased by Sebastian and made the default value to be unicode.

Reviewed-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

mediagoblin/submit/views.py

@@ -41,7 +41,8 @@ def submit_start(request):
     """
     First view for submitting a file.
     """
-    submit_form = submit_forms.SubmitStartForm(request.form)
+    submit_form = submit_forms.SubmitStartForm(request.form,
+        license=request.user.get('license_preference'))
 
     if request.method == 'POST' and submit_form.validate():
         if not ('file' in request.files