Implement acorn checks for GWSumm JavaScript

[alurban]

This PR implements an additional build-and-test phase for JavaScript elements using acorn, and for CSS elements using stylelint and standard SASS guidelines. The gwsumm.scss source file is corrected for a number of linting complaints.

This also explicitly requires flake8 and pytest-cov from the setup.py script.

cc @duncanmmacleod

[codecov]

Codecov Report

Merging #297 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #297   +/-   ##
=======================================
  Coverage   38.89%   38.89%           
=======================================
  Files          47       47           
  Lines        7529     7529           
=======================================
  Hits         2928     2928           
  Misses       4601     4601

Flag	Coverage Δ
#Linux	38.89% <ø> (ø)	⬆️
#python35	38.86% <ø> (ø)	⬆️
#python36	38.89% <ø> (ø)	⬆️
#python37	38.89% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 054f603...0fa9227. Read the comment docs.