base: folder refactoring

* Moves fixtures and data from base/ to demosite/ folder.

* Updates README accordingly.

Signed-off-by: Javier Martin Montull <javier.martin.montull@cern.ch>
base: port old KBs to overlay

* Creates new knowledgeext/kb/ folder where all KB files
  are stored.

* Creates fixture classes on the fly based on KB file
  content.

* NOTE Using $ inveniomanage database recreate
  will load all KBs into the database which is a one-off slow process.

Signed-off-by: Javier Martin Montull <javier.martin.montull@cern.ch>