Support link-checker ignore attribute for last-mod and page-meta links

[chalin]

Some link-checking tools support ignore attributes, like data-proofer-ignore:

• html-proofer > Ignoring content
• htmltest > Ignoring content

For external links generated from known sources, such as GitHub last-modified link, we should support use such ignore attributes. It might be useful to apply such attributes to other links too (page edit?), but this will require more thought (since misconfigured page-edit links can be invalid).

• For last-mode links - External link checking + project pages #2120
• For page-meta (i.e., repo) links

[chalin]

Closed by:

• External link checking + project pages #2120

[chalin]

Reopening so as to track the addition of this feature for page-meta links too.