Add model for storing exported git URL (issue #207)

trans/models/subproject.py

@@ -94,6 +94,11 @@ class SubProject(models.Model, PercentMixin, URLMixin):
         validators=[validate_repoweb],
         blank=True,
     )
+    git_export = models.CharField(
+        max_length=200,
+        help_text=ugettext_lazy('URL of Git repository where users can fetch changes from Weblate'),
+        blank=True
+    )
     report_source_bugs = models.EmailField(
         help_text=ugettext_lazy(
             'Email address where errors in source string will be reported, '