New UI for ubcpi

[kitsook]

UI revision:

• Revise UI to improve visual consistency
• Fix problem of response missing when switching between units
• Add test cases for new chart format
• Generate and return peer answers on-the-fly if nothing persisted

Closes #144
Closes #148

Fix issues with deleting learner state:

• implement the callback clear_student_state
• add a new submission to indicate existing answers are deleted
• add (but commented out) code to partially support revising score manually. But full support will require adding logic to persist the score within xblock
• fix issue with same explanation displayed multiple times in Step 2
• simplify the Simple algo for picking sample explanation. Instead of using arbitrary max loop count of 100 and hope to pick unique explanations, remove item from the pool once they are chosen.

Closes #119
Closes #149

Allow students to fresh rationales shown

• on step 2, display a link to allow students to refresh the answers shown

Closes #165

[coveralls]

Coverage decreased (-1.3%) to 95.365% when pulling 32ba633 on students-req-more-answers into 412c114 on master.

[lenglund]

Great! Just need to update the background for the answer boxes around the graphs to #fbfbfb.

Two other thoughts:

1. Can we have the "Next Step" button on the first step and "Submit Answer" button on the second step advances AND scroll to the progress bar (the box with the Answer - Reflection - Results)? This would help a lot with orienting after each change and hopefully address student issues with missing aspects of the UI (as reported in the survey data).

2. Is it possible to disable (i.e., gray out--#888--and make not clickable) the other samples option when there are no other samples to show? Otherwise, the feature looks broken, saying it's refreshing but then nothing changes.

Thanks!

[kitsook]

@lenglund Changes deployed to testing env.

Note that the logic to enable/disable the refresh samples button is not 100% accurate, e.g. instructors may have deleted the answer or answers got removed from the pool by the time students click on the refresh button. So there is always a chance to see the same samples again after clicking the refresh button.

[lenglund]

Sounds good and LGTM! Thanks for your work on this.