Allow explicit msg id to be passed to show_message.
Include fixed block of 5 questions after completing streak of 8/10.
Don't display empty messages in show_message.
add assert to make sure we have Nalanda package activated
Merge branch '2167' of https://github.com/aronasorman/ka-lite into fixed_exercise_block
Merge branch '2167' of https://github.com/aronasorman/ka-lite into fixed_exercise_block

Conflicts:
	kalite/distributed/views.py
JS template for A/B testing, and make FIXED_BLOCK_EXERCISES changeable.
Merge branch 'nalanda-rct2' of github.com:learningequality/ka-lite into fixed_exercise_block
Merge pull request #2201 from jamalex/fixed_exercise_block

Include fixed block of 5 questions after completing streak of 8/10 (addressing #2168)