Added support for preview links #2184

[seagulley]

The PR fulfills these requirements: (check all the apply)

• It's submitted to the main branch.
• When resolving a specific issue, it's referenced in the PR's title (e.g. feat: Add a button #xxx, where "xxx" is the issue number).
• When resolving a specific issue, the PR description includes Closes #xxx, where "xxx" is the issue number.
• If changes were made to ui folder, unit tests (make test) still pass.
• New/updated tests are included

Added new variables to conf.go ServerConf: PreviewImage, PreviewTitle, PreviewDescription.
Modified web_server.go to use these new values as meta tags in the header of the root index.html.

Closes #2184

[mturoci]

Thanks for working on this @seagulley! I would like to see some demo to test these changes in action.

[mturoci]

Make these part of the https://github.com/h2oai/wave/blob/94bf37f538719ad279a41b0df1b452623d61aaa4/conf.go#L71C7-L71C7 as well to expose new options to the users.

Also document here.

[seagulley]

I'll get started on this as well as a demo :)

[seagulley]

Hi Martin,

I started working on the demo. Using Meta's Open Graph debugger, it can read the custom title and description, but there are some issues with the image.

I did some more digging and there are some guidelines for adding images to preview links
https://developers.facebook.com/docs/sharing/best-practices/#precaching

How should I approach this?

[mturoci]

The warning says all it needs are og:image:height and og:image:width props. I would first try it out to see if they help. If they do, the next step would be to validate passed image dimensions and:

• If the image does not adhere to the recommended ratio or is too small, log warning and use the actual dimensions for the respective props.
• If the image ratio is good and >= required dimensions, use the copy of the image resized to the expected dimensions.

Hope that makes sense.

[seagulley]

Yes that makes sense I'll get started on it!

[mturoci]

nit: Should be enough.

Suggested change

	func mungeIndexPage(baseURL, html string, previewImage string, previewTitle string, previewDescription string) string {
	func mungeIndexPage(baseURL, html, previewImage, previewTitle, previewDescription string) string {