[webui] Bring back :service mapping for activexml for remote add_file
[webui] add test case for 'remove file'

(and move the functionality in the package model)
[webui] remove dead service model code
[webui] remove the rest of the engine

also make sure the API routes to not try to reply to HTML - which only
gives crashes
[webui] global catch for invalid :back

if people visit invalid URLs without coming from links of ours, then
they just get the main page.