Feature summary (what you would like to be able to do and where):
When using FlaggedRevs the “German style” (all pages in the given namespaces should be reviewed, in contrast to the “English-style” protection-based mode), pages not yet reviewed are likely to be of lower quality. There should be a configuration option to exclude them from possible targets of Special:Random. (Implementation notice: this can be done using the SpecialRandomGetRandomTitle hook.)

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
Being able to disallow Special:Random to jump to unreviewed pages.

Benefits (why should this be implemented?):
Readers are more likely to get high-quality content when using Special:Random.