Take advantage of event listeners in gwbootstrap

[alurban]

This PR takes advantage of a couple new event listeners added to gwbootstrap for button click actions, including for table-to-CSV conversion and the return-to-top button.

See also gwdetchar/gwbootstrap#9.

cc @duncanmmacleod

[codecov]

Codecov Report

Merging #300 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #300   +/-   ##
=======================================
  Coverage   39.09%   39.09%           
=======================================
  Files          47       47           
  Lines        7562     7562           
=======================================
  Hits         2956     2956           
  Misses       4606     4606

Flag	Coverage Δ
#Linux	39.09% <100%> (ø)	⬆️
#python35	39.06% <100%> (ø)	⬆️
#python36	39.09% <100%> (-60.91%)	⬇️
#python37	39.09% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
gwsumm/tabs/gracedb.py	13.5% <ø> (ø)	⬆️
gwsumm/tabs/guardian.py	19.05% <ø> (ø)	⬆️
gwsumm/tabs/sei.py	15.06% <ø> (ø)	⬆️
gwsumm/html/bootstrap.py	100% <100%> (ø)	⬆️

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