Switch log snapshot from file path reference to UUID

[AFaust]

CHECKLIST

• There aren't existing pull requests attempting to address the issue mentioned here
• Submission developed in a feature branch--not master

CONVINCING DESCRIPTION

This PR changes the way the log snapshot feature deals with identifying a particular snapshot session. Instead of using the full file path for reference (and having to validate it), a generic UUID is now being used. This avoids any encoding issues e.g. when the file path may contain backslashes instead of regular slashes.
In addition to this change, various smaller aspects have also been addressed:

• move the temporary file handling into the temporary appender itself
• move the appender layout default into the temporary appender itself
• switch the snapshot start web script to use proper HTTP method (POST) - note that completion web script could not be switched to proper HTTP method as it is required to be a GET for use in the web UI

RELATED INFORMATION

Fixes #91

Note: By editing the Java-backed web scripts my formatter / auto-save rules have been applied to yield a consistent style. We still have #83 to further consolidate, document style conventions and potentially provide tools to achieve consistent code style.