Preparation to code split by route and measure impact

[kmjennison]

• Add a built-in measurement of "time to interactive" using Google's TTI polyfill
• Update react-router-dom to a beta to fix a bug with React's lazy
• Add source-map-explorer to inspect our bundles
  • We may want to try others, too, like Bundle Buddy, etc.
• Move Firebase app initialization to root.js
• Move Route.js into root.js for simplicity

[codecov]

Codecov Report

Merging #457 into master will decrease coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #457      +/-   ##
==========================================
- Coverage    53.1%   53.05%   -0.05%     
==========================================
  Files         250      249       -1     
  Lines        6783     6789       +6     
  Branches     1190     1191       +1     
==========================================
  Hits         3602     3602              
- Misses       2518     2523       +5     
- Partials      663      664       +1

Impacted Files	Coverage Δ
web/src/index.js	0% <ø> (ø)	⬆️
web/src/js/root.js	0% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[codecov]

Codecov Report

Merging #457 into master will decrease coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #457      +/-   ##
==========================================
- Coverage    53.1%   53.05%   -0.05%     
==========================================
  Files         250      249       -1     
  Lines        6783     6789       +6     
  Branches     1190     1191       +1     
==========================================
  Hits         3602     3602              
- Misses       2518     2523       +5     
- Partials      663      664       +1

Impacted Files	Coverage Δ
web/src/index.js	0% <ø> (ø)	⬆️
web/src/js/root.js	0% <0%> (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 b7e4b6c...1bd113b. Read the comment docs.