Add activity log

[MasterZydra]

Add logging of user activities in file (one file per day) or database table.
After a defined time the old log files will be deleted (see #55 for Cron).

Log table:

• Time stamp
• Username
• Type (Error, Warning, Info, User activity)
• Message (Text)
  • Module (Plot, Invoice, ...)
  • Action (Edit, Add, Delete, ...)
  • Change as JSON?

As administrator there is a block in the administrator area where the files are listed and can be viewed.