Add some tolerance to the autoscrolling feature of taillog

[DL6ER]

What does this implement/fix?

On initial load of the taillog page, the "autoscroll" feature is enabled so you will always see the most recent log events. If something catches you attention you can stop this by simply scrolling a bit up. You can then re-enable autoscroll by scrolling again down to the very bottom.

So far, so good. This seems to work reasonably well. however, you have to scroll down all the way because the textbox must be at the exact bottom for autoscrolling to be re-enabled. This PR improves this behavior by adding a tolerance of four line heights for autoscrolling to be re-engage.

This PR is correlated to this Discourse post (German).

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Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

Checklist:

• The code change is tested and works locally.
• I based my code and PRs against the repositories developmental branch.
• I signed off all commits. Pi-hole enforces the DCO for all contributions
• I signed all my commits. Pi-hole requires signatures to verify authorship
• I have read the above and my PR is ready for review.

[DL6ER]

Confirmed to be working on Discourse

[yubiuser]

Thanks for adding a detailed comment section.