Commonize the HTML rendering for OAuth install path

[seratch]

Summary

This pull request improves the internals of built-in receivers by commonizing duplicated code across the implementations.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #810 (9cacbe8) into main (f722fe8) will increase coverage by 0.12%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main     #810      +/-   ##
==========================================
+ Coverage   65.47%   65.59%   +0.12%     
==========================================
  Files          11       12       +1     
  Lines        1089     1093       +4     
  Branches      323      323              
==========================================
+ Hits          713      717       +4     
  Misses        317      317              
  Partials       59       59              

Impacted Files	Coverage Δ
src/receivers/ExpressReceiver.ts	66.87% <50.00%> (+0.20%)	⬆️
src/receivers/HTTPReceiver.ts	17.15% <50.00%> (+0.59%)	⬆️
src/receivers/SocketModeReceiver.ts	11.53% <50.00%> (+1.73%)	⬆️
src/receivers/render-html-for-install-path.ts	50.00% <50.00%> (ø)

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