private-webmention and ACL

[elf-pavlik]

conversation for https://indiewebcamp.com/private-webmention

if resource server hosting private resource (webmention source) has an rel="acl" resource for that source resource, those who can read this acl resource can also see who already has access to it and possibly can modify that resource to add new access grants

this acl resource could also list shareable links which use Capability URLs and allow their revocation.

@fkooman @melvincarvalho @csarven

See also:

• Webmention for private content #12 Webmention for private content
• Consider adding webkey support #36 Consider adding webkey support
• https://www.w3.org/wiki/WebAccessControl
• https://github.com/linkeddata/SoLiD#access-control
• https://www.w3.org/wiki/WebID/Authorization_Delegation