Fix #982 Enable developers to customize the "/slack/install" webpage content

[seratch]

Summary

This pull request fixes #982. Refer to the issue for details.

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 #1083 (453d3f8) into main (926b669) will decrease coverage by 0.05%.
The diff coverage is 73.33%.

❗ Current head 453d3f8 differs from pull request most recent head ac6b5bb. Consider uploading reports for the commit ac6b5bb to get more accurate results

@@            Coverage Diff             @@
##             main    #1083      +/-   ##
==========================================
- Coverage   71.01%   70.95%   -0.06%     
==========================================
  Files          13       13              
  Lines        1235     1243       +8     
  Branches      362      365       +3     
==========================================
+ Hits          877      882       +5     
- Misses        288      291       +3     
  Partials       70       70              

Impacted Files	Coverage Δ
src/receivers/ExpressReceiver.ts	64.88% <25.00%> (-0.79%)	⬇️
src/receivers/render-html-for-install-path.ts	75.00% <66.66%> (-25.00%)	⬇️
src/receivers/HTTPReceiver.ts	32.77% <100.00%> (+0.75%)	⬆️
src/receivers/SocketModeReceiver.ts	69.49% <100.00%> (+1.07%)	⬆️

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[seratch]

This one is ready for merge but let's hold off merging until we merge #1024

[seratch]

Resolved conflicts ✅