[etwist] clear ui cache at server startup time. Closes #1806933
this avoid pb when cubicweb/cubes provide new version of some js/css and also accumulating junk in the cache directories
Schema of the data
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First take a look at the data schema_ then try to remember that you are browsing
through a heap of data by applying stylesheets to the results of queries. This
site is not a content management system with items placed in folders. It is an
interface to a database which applies a view to retreived data.
.. _schema: schema