Fix feature importance

[tweddielin]

I've been struggling with different charting js libraries. So far we've used Highchart.js and react-nvd3. Highchart is good, but it's a little be overkilled. react-nvd3 is a react wrapper of nvd3 which I thought it's pretty handy in the beginning, but it turned out not really flexible when we want to adjust some lower-level parameters in the graph and it doesn't look like there's someone actually maintaining it anymore. Then I found there's a new library that uber recently developed called react-vis. It's pretty easy to use and it's a pretty active project in uber. The feature importance plot is now using react-vis. I've also looked into other libraries like victory, rumble-charts and react-d3-components. There are so many charting libraries now, but eventually we'll have to choose just one library.

• Added a num of features variable to feature importance endpoint route.
• Added a dropdown menu to choose the number of features to show in feature importance
• Use react-vis for feature importance graph
• Slightly changed the layout

[codecov-io]

Codecov Report

Merging #60 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #60   +/-   ##
=======================================
  Coverage   71.28%   71.28%           
=======================================
  Files           4        4           
  Lines         303      303           
=======================================
  Hits          216      216           
  Misses         87       87

Impacted Files	Coverage Δ
webapp/controller.py	61.02% <100%> (ø)	:arrow_up:

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