Implements a new exception class for webui : HttpErrors that uses a new template : errors.html to display multiples errors

They were a bug when ordering stuff contains 2 values that are equals

- a "normal" import was left in lefactory_common
- the hook triggering datasources init in dyncode was not called
- dyncode was not exposed in the loader (making hook call useless : the hook has to be called once the dyncode has been "seen" (imported) by python)

Regression may have been introduced about MONOSITE mode

+ uses the changes in the preload method

- found a way to make it implements the socketserver.ForkinTCPServer features reducing the global code size (and removed a lot of dirty copy & paste from cpython sources)
- deleted useless imports

- updated scripts etc
- make tests pass

- I do not understand wy the set test for Settings was OK before...