Rework the behavior of favicon search

[jaden]

Closes #5838

Changes proposed in this pull request:

• Use xpath queries to reduce the filtering necessary.
• Don't call checkUrl since the href value will not always be a full URL.
• Check for a base path in the HTML to be used if the favicon href doesn't start with a leading slash.

How to test the feature manually:

1. Subscribe to sites with favicon link hrefs that don't include the domain.
2. Verify that the correct favicon is loaded.

Pull request checklist:

• clear commit messages
• code manually tested
• unit tests written (optional if too hard)
• documentation updated

Additional information can be found in the documentation.

[Alkarex]

Thanks! Looks good.
Try to run make test-all

[Alkarex]

Good improvement 👍🏻
Please add a line for you https://github.com/FreshRSS/FreshRSS/blob/edge/CREDITS.md