Add additional features to the UnReview dataset for improving recommendations

To improve UnReview recommendations, we might consider adding additional features when making recommendations:

• file types, a.k.a. RR categories, to give different recommendations for different predefined groups of files (e.g., docs, backend, frontend)
• time of the last commit/review to filter out outdated recommendations (e.g., a person left the company)

The new features proposed still keep UnReview independent from the customer's project.

cc @mray2020 @whaber @a_akgun @phikai @tmccaslin