CSS includes are hard-linked to releases.u.c instead of docroot-relative paths

[Thomas Ward]

Related RT tickets:
#32732 and #32842

The CSS includes used by the cd images generators are hard-linked to releases.ubuntu.com/ or similar. This results in the breakage of CSS includes on HTTPS:// enforced/enabled mirrors, whether they be local or remote, because releases.ubuntu.com does NOT have an HTTPS:// listener on them.

In RT #32732 the root pages for the HEADER/FOOTER on a mirror were updated to be site docroot-relative, rather than pulling from releases.u.c. This change was implemented and is able to eb seen across all existing releases.u.c mirrors.

However, it was discovered that this doesn't happen for the individual cd images' HEADER/FOOTER HTML pages. This means that CSS includes for those pages *fail* when on an HTTPS:// mirror. (RT #32842).

This needs adjusted in the code to address this and to use docroot-relative includes for CSS rather than hardlinking to releases.u.c.

(This bug was created in relation to a merge proposal which partially fixes this problem)