[backend] remove $conf as payload for SRCSRV_START

No notification plugin can handle perl code as type, nor is it supposed
to handle it. I wonder who is interested in this event anyway
[api] also allow render exceptions in JSON routes
[backend] remove old request handling

Only leaving the functions needed to run the 2.4 API migrations
[api] remove some files not needed anymore
[api] fix structure.sql
[webui] fix spider test suite