Add analytics

[thostetler]

• add react-gtm-hook
• add analytics call for theme change
• Add analytics call for search example click
• add analytics call for facets

[codecov]

Codecov Report

Merging #346 (fce662b) into master (52142e0) will decrease coverage by 0.06%.
The diff coverage is 11.86%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #346      +/-   ##
==========================================
- Coverage   47.89%   47.83%   -0.06%     
==========================================
  Files         392      392              
  Lines       40555    40611      +56     
  Branches      558      558              
==========================================
+ Hits        19422    19428       +6     
- Misses      21121    21171      +50     
  Partials       12       12              

Files	Coverage Δ
src/components/SearchFacet/SearchFacet.tsx	16.90% <14.28%> (-0.05%)	⬇️
src/components/SearchExamples/SearchExamples.tsx	21.42% <12.50%> (-0.80%)	⬇️
src/components/AllAuthorsModal/AllAuthorsModal.tsx	19.62% <7.69%> (-0.44%)	⬇️
...ponents/CitationExporter/components/ResultArea.tsx	55.84% <18.75%> (-9.73%)	⬇️
src/components/CopyButton/CopyButton.tsx	22.03% <6.66%> (-4.06%)	⬇️

... and 1 file with indirect coverage changes