Logs backends messages in specific log files

[rezib]

Currently, the backend uses basic print() and the default flask/werkzeug logging mechanism to report errors on stderr and HTTP error logfile. Then, application errors pullute quite a lot HTTP servers logs...

We should introduce a new mechanism to report logs in application specific logfile, quite similar to what's been done in jobmetrics:

https://github.com/edf-hpc/jobmetrics/commit/9172f809dda329499b55ee4dbdcc96d2efb3d743