luci-app-privoxy: cleanup and fixes for #298 #334 #335

* remove extra apperror.lua page
* so no nore need for extra directory inside model/cbi
* add option temporary_directory needed for external filters
* remove special handling of bootstrap theme #298 #334
* modified version handling suggested in #335

Signed-off-by: Christian Schoenebeck <christian.schoenebeck@gmail.com>