ScriptConsole activity is not logged

https://github.com/jenkinsci/jenkins/pull/4734

Jenkins does not log script console activity. This pull request implements logging in the following format:

IP|username|Activity Type (in this case, "ScriptConsole")|Additional context (in this case, Base64 of the script executing).

Log severity is set to INFO. This is the lowest severity that is still shown in the logs without setting up custom listeners. This is by design, so that all Jenkins installs will have this log without having to modify the installation.

The log fields are split by the pipe character for easy parsing, and the script contents are Base64 encoded to ensure the log is in a single line of text.