perf(image): replace js images lazy loading by html loading attribute

[sad270]

Closes #2371

Changes proposed in this pull request:

• Replace js image lazy loading by loading="lazy" html attribute
• Add loading="lazy" on all rss content image if lazy-loading configuration is true (even if the "display_post is enabled)

How to test the feature manually:

1. Enable Lazy loading from setting
2. Open a feed with images
3. Check if loading="lazy" attribute is set
4. You can also open "Network" tab of your browser developer options and check if the feed images is only load when the feed is open, and on scroll.

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]

We need to check upstream that we filter out existing loading attributes (that might already be the case), to make sure we do not end up with two identical attributes, which would be invalid

[Alkarex]

This removes the option of not having lazy loading, which is not good

[Alkarex]

Thanks for the PR!
The use-case of not using lazy-loading must be preserved (descriptions of the need can be found in past issues, for instance on slow connections, to make sure all images are loaded before starting to read)