Show a tooltip when the mouse hovers over a bar

[noisecapella]

What are the relevant tickets?

Fixes #47

What's this PR do?

Adds a tooltip that shows the information about the answer when the mouse hovers over the bar.

How should this be manually tested?

Move the mouse cursor over various bars

[codecov-io]

Codecov Report

Merging #57 into master will increase coverage by 0.1%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master      #57     +/-   ##
=========================================
+ Coverage    89.9%   90.01%   +0.1%     
=========================================
  Files          15       15             
  Lines         555      561      +6     
  Branches       32       34      +2     
=========================================
+ Hits          499      505      +6     
  Misses         51       51             
  Partials        5        5

Impacted Files	Coverage Δ
rapid_response_xblock/block.py	99.05% <100%> (ø)	:arrow_up:
tests/test_aside.py	100% <100%> (ø)	:arrow_up:

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[noisecapella]

I refactored the tooltip template code. The mousemove handler now only adjusts position to avoid possible performance issues with rendering the template repeatedly

[noisecapella]

Also this is now rebased on #54, Travis is slow so it's taking a while to merge

[noisecapella]

I addressed the comment, I also made a change to fix disabled button handling.

[noisecapella]

I made the div initialization change