Gravatar business cards don't work on posts loaded with infinite scroll

#### Steps to reproduce the issue

1. Switch to a theme that outputs gravatar for posts with enough posts. (alternatively you can add `` to your loop)
2. Enable infinite scrolling.
3. On the homepage, hover over avatars and see business cards hover.
4. Load a second page of posts using the infinite scroll button.
5. Hover over the newly loaded avatars.
6. No business cards will pop up

#### What I expected

Business cards should still work on new posts loaded with infinite scroll

#### What happened instead

The business cards didn't show up

Relevant discussion on this PR #14922. After that, the script is not loaded by default. So the fix must take that into account

Start by reviewing the infinite-scroll loading path, the get_avatar() usage shown in the report, and the discussion in PR #14922, especially the change where the script is no longer loaded by default. Reproduce the homepage case with a second page of posts and verify that business cards appear for newly loaded avatars without breaking the default script behavior.

Written by the indexing model from the issue text.