feat: include URLs in GitHub app and secret suggestions

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🚀 Feature: "enable the GitHub apps" / "populate the secrets" to include links to the relevant URLs  #1621
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Refactors the two blocks to use a shared getInstallationSuggestions function because I didn't want to keep updating both.

🎁

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.55%. Comparing base (92ef8c8) to head (774a865).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1964      +/-   ##
==========================================
+ Coverage   95.48%   95.55%   +0.06%     
==========================================
  Files          90       91       +1     
  Lines        3635     3643       +8     
  Branches      214      214              
==========================================
+ Hits         3471     3481      +10     
+ Misses        164      162       -2     

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[github-actions]

🎉 This is included in version v2.4.0 🎉

The release is available on:

• GitHub releases
• npm package (@latest dist-tag)

Cheers! 📦🚀