Bugfix #2920 Avoid doing unecessary calls to Amazon Web Services

Bugfix #2920 Avoid doing unecessary calls to Amazon Web Services

This patch modifies how AWS is called. AWS is now called
depending on syspref. It works completly for OPAC; it
has to be refined for intranet.

For OPAC:

  * If OPACAmazonReviews is set, AWS EditorialReview and
    Reviews (users) are grabed.
  * If OPACAmazonSimilarItems is set, AWS Similarities info
    are grabed.
  * If nothing is asked, AWS is not called anymore,
    sparing server bandwidth.

For intranet:

  It works as it used to work. AWS is called if AmazonEnabled
  is set whatever how other syspref are set.

  TODO:

    * Add a AmazonReviews syspref
    * Request Amazon content depending on AmazonSimilarities
      and AmazonReviews syspref

DOCUMENTATION:

  It should be explained that Amazon services related syspref
  have two levels:

  * AmazonEnable / OPACAmazonEnable
  * Other: OPACAmazonReviews (new), OPACAmazonSimilarProduct,
    OPACAmazonCover

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>