Add global support for local added content files.

Add global support for local added content files.

If files exist under (by default) /openils/var/web/opac/extras/ac/{type}/{format}/{key}
on the content server (the main server, unless a separate content server is configured),
these will be served in preference to remote added content.  The files here should contain
the same type of data that would be expected from a remote added-content engine, such as
images or html pages.

  * "type" is one of jacket, reviews, toc, excerpt or anotes
  * "format" is type dependent:
    - for jacket, one of small, medium or large
    - others, one of html, xml or json ... html is the default for non-image added content
  * "key" is the sanitized ISBN (any leading spaces are removed, then anything after the
    first space is removed)

git-svn-id: svn://svn.open-ils.org/ILS/trunk@15105 dcc99617-32d9-48b4-a31d-7c20da2025e4