Import / export of the dashboards. (#1197)

* Implement import / export dashboard functionality.

* Address comments from discussion.

* Add function descriptions.

* Minor fixes

* Fix tests for python 3.

* Export datasources.

* Implement tables import.

* Json.loads does not support trailing commas.

* Improve alter_dict func

* Resolve comments.

* Refactor tests

* Move params_dict and alter_params to the ImportMixin

* Fix flask menues.

tests/core_tests.py

@@ -452,6 +452,5 @@ class CoreTests(CaravelTestCase):
         db.session.commit()
         self.test_save_dash('alpha')
 
-
 if __name__ == '__main__':
     unittest.main()