[META] Support subsites / subdirectory sites like `example.com/foo` (Part III)

Previous to this most recent push, we've had the following meta issues:

1. #705026: Allow creation of example.com/foo and example.com/bar type of sites (Part I)
2. #2046167: [META] Remaining issues with subsite support (Part II)

This meta issue represents the latest batch:

• #3068595: hosting_subdirs fails to clean up subdir.d folder when sites are deleted
• #3036884: Use redirect target for site title in UI
• #3040392: Ensure that subdirectory sites work properly with Let's Encrypt
• #3036890: Simplify subdir site installation
• #3068259: Cannot rename non-subdirectory site to subdirectory site
• #3068257: Migrating a subdirectory site reverts to internal domain & loses alias
• #3034235: Use alias redirection target when creating goto/login link
• #3025905: Login URLs not working on Drupal 8 subdirectory sites
• #3066538: Cookie subdomains missing first part of subdomain for non-subdirectory sites
• #3066146: Automatically spawn a Verify task for parent on subdir site installation
• #3014381: Logins impossible on subdirectory sites