keep some history of planning engine log files

[jdetaeye]

Currently the log file of a planning run overwrites the previous one. Keeping the X last files would be useful in some cases.

Implementation:

• add a timestamp to the filename
• use the Python logging capabilities to capture the content to be logged. Clean and consistent approach, better than ad-hoc print statements.
• execute screen: the current "view log file" button needs to be put next to a plan generation record
• keep at most X log files. Or max size? Or max duration? Or the most constraining of these 3?

[jranita]

Timestamp added. View log file removed and replaced by button in grid. Log files limited by total size parameter in djangosettings, deletion occurs in frepple_runworker.

Still missing Python logging instead of prints.