+ http: Add model for `Link` header

This adds basic support for the Link header as described by RFC5988, Section 5:
http://tools.ietf.org/html/rfc5988#section-5

Multiple link-values are supported, but of the link-params only the rel, title, and anchor params are supported.
= http: ignore additional `rel` LinkParams after the first, refs: #609
= http: refactor new Link header model

Even though this change appear to be breaking it is neutral with respect to the last release
Merge branch 'stig-feature/link-header'
= site: update `community-projects` page
= routing: improve CookieDirectivesSpec
+ httpx, routing: distinguish between malformed and invalid content for deserialization errors