use javascript to open new tab for external links

[cosmicBboy]

This PR removes the sphinx-new-tab-link dependency for adding target="_blank" to external links in favor of using a simple custom javascript file to achieve the same effect.

Why are the changes needed?

The sphinx-new-tab-link dependency caused an unintended change in the html/css styling of the site (black border on table elements):

sphinx-new-tab-link depends on sphinxcontrib-extdevhelper-kasane, (repo), which seems to modify the way the html docs are rendered in a non-obvious way.

What changes were proposed in this pull request?

Use sphinx's supported way of customizing docs with a custom.js file that achieves the same thing, but without the additional python dependency.

How was this patch tested?

Docs built locally with make docs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.11%. Comparing base (c81133b) to head (a0aaec9).
Report is 14 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5159      +/-   ##
==========================================
+ Coverage   59.03%   59.11%   +0.08%     
==========================================
  Files         645      645              
  Lines       55726    55574     -152     
==========================================
- Hits        32896    32851      -45     
+ Misses      20233    20130     -103     
+ Partials     2597     2593       -4     

Flag	Coverage Δ
unittests	59.11% <ø> (+0.08%)	⬆️

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