Add a global configuration page for Prism

[uhafner]

These configuration options are used globally for all jobs and require administrator permissions.

The following settings can be configured:

• Allowed source code directories: some plugins copy source code files to Jenkins' build folder so that these files can be rendered in the user interface together with build results (coverage, warnings, etc.). If these files are not part of the workspace of a build then Jenkins will not show them by default: otherwise sensitive files could be shown by accident. You can provide a list of additional source code directories that are allowed to be shown in Jenkins user interface here. Note, that such a directory must be an absolute path on the agent that executes the build.
• Theme: Prism supports several themes that can be used to adapt the look and feel. You can configure the
  default theme that is used for all Jenkins jobs.

Besides the global configuration, also classes and forms are available to provide a configuration of these values in a job.