Add notes and author text boxes to keep track of settings modifications

[jaferrer71]

It would be usefull to have a textbox added for some configurations that allows to enter some notes to that entry, and also keep record of who did it. For example in Settings :: MTA Settings :: DNSBL Checks Exemptions, you can just add an IP address, so it would be great to know why it was added and by who. This is also valid for other configurations as other exemptions list